Psychosocial Interventions and Support Groups for Siblings of Individuals with Neurodevelopmental Conditions: A Mixed Methods Systematic Review of Sibling Self-reported Mental Health and Wellbeing Outcomes

No systematic review to date has identified and reviewed the risk and resilience-promoting factors characterising siblings who may benefit more than others from interventions in the included studies. Further, siblings’ qualitative experiences have not been systematically considered and no prior review has incorporated sibling community consultation. None of the reviews to date specifically focussed on siblings’ self-reported mental health and/or wellbeing outcomes following interventions. Self-report is integral to designing effective and tailored interventions according to siblings’ qualitative and unique experiences (Houtzager et al., 2005), particularly as parents have been found to both under- and over-report on siblings’ wellbeing and emotional adjustment, leading to reporting bias and overlooked sibling needs (McKenzie Smith et al., 2018; Rankin et al., 2017).

Therefore, the present review aimed to address the above limitations and conduct an up-to-date systematic review of intervention and support group studies of siblings of individuals with NDCs, focussing on self-reported outcomes, including both quantitative and qualitative studies and incorporating a sibling community consultation.

The following research questions were explored:

Before commencing, other systematic review databases (Cochrane Library, Prospero, JBI EBP, DoPHER) were searched for similar registered reviews and no searches produced systematic review protocols involving the key terms for this review. Databases searched included: Ovid MEDLINE, Embase, CINAHL, PsycINFO, PsycARTICLES, Pubmed, ProQuest, Web of Science, Scopus and Google Scholar.

A pilot search was conducted. Search strings were identified and run which maximised sensitivity and minimised the absolute difference between sensitivity and specificity. We allowed higher sensitivity than specificity to ensure all/most relevant records were found and the best search string optimisation achieved 91.1% sensitivity and 83.2% specificity, with a precision of 5.1%. The search was limited to titles and abstracts. The search terms included database specific controlled vocabulary, field codes, operators, relevant keywords and subject headings to identify the population of interest (siblings), the exposure (psychosocial interventions) and the outcome (mental health or wellbeing). A comprehensive list of search terms to encompass mental health outcomes following interventions for siblings of persons with NDCs were included and these terms are presented in full in Table 1 of the Appendix. Search terms in each category were combined using the Boolean operator ‘OR’; search categories were combined using the Boolean operator ‘AND’. The final search strategy and the specific search terms are presented in Table 1 (Appendix).

The first author (BW) screened all identified records by reading titles and abstracts, excluding abstracts clearly not meeting the eligibility criteria and then reading full texts after this initial screening. The first author (BW) then decided on final records for inclusion based on the meeting all of the above eligibility criteria; any studies with questionable eligibility were screened by the last author (EG) and discussed to reach an agreement on inclusion. Further, a random 20% of full texts identified were screened by the last author (EG) and inter-rater agreement was very good (85%). Any disagreements were discussed and resolved. The final list of records was read by the last author (EG), following which forwards and backwards citation searching was conducted for all eligible articles, leading to a further six articles included. These six studies met all eligibility criteria described above and were agreed upon by the first and last authors.

The most common reasons for exclusion of articles were: incorrect or insufficient intervention/programme outcome data; the sibling disability not falling within our definition of an NDC; the study not being an intervention or support group design; or the study focussing on an individual other than the sibling (e.g. the parents or a sibling-mediated intervention designed for the sibling with an NDC; see Fig. 1).

A standardised form used to extract data from the eligible studies was piloted on a validation dataset. Search results were exported to EndNote citation manager and duplicates were removed. Full-text papers were read by two study investigators independently (BW, EG). No automaton tools were used.

The first author (BW) extracted all data items (see Tables 2 and 3), which were then cross-checked by the last author (EG).

A meta-analysis of effect sizes was not carried out due to the large heterogeneity in measures used, intervention designs and sibling sample characteristics. The body of evidence across sibling-reported mental health/wellbeing outcomes were coded as overall positive (a beneficial effect of the intervention), overall negative (a negative effect of the intervention) or neutral (no significant difference pre–post-intervention and/or between treatment and control groups post-intervention).

Risk of bias in individual studies was assessed by two of the authors (BW, EG) using the Mixed Method Appraisal Tool (MMAT; Hong et al., 2018). Each study received one point for five items (Table 4); the number of items endorsed indicated the study quality: ‘high’ (4 or 5 of 5 indicators endorsed), ‘moderate’ (3 indicators) or ‘low’ (0–2 indicators).

Data synthesis and integration first followed the Joanna Briggs Institute Manual for Evidence Synthesis convergent segregated approach (Aromataris & Munn, 2020). This involved initial separate quantitative and qualitative integration, followed by the convergent integrated approach (Aromataris & Munn, 2020; Hong et al., 2018), which “qualitised” quantitative data using thematic analyses. This was conducted using the accepted standard of thematic synthesis, beginning with coding of text line-by-line, then developing descriptive themes and subsequently developing analytical themes which generated new interpretive constructs or explanations; Thomas & Harden, 2008). The qualitised data was then pooled with the qualitative data to produce overarching analytical themes encompassing all studies (Thomas & Harden, 2008). The themes were initially developed by the first author (BW) and discussed, reviewed and agreed upon with the second and last authors (IM, EG). These analytical themes provided an integrated framework for interpreting the mental health/wellbeing outcomes across all studies.

The Cochrane tool for the Grading of Recommendations Assessment, Development and Evaluation (GRADE; Guyatt et al., 2011) was used to assess risk of bias. A ranking of high, moderate, low or very low certainty based on the pooled evidence is provided in Table 6.

The consultation aimed to seek and obtain verbal and written feedback on the findings of the current review and to obtain their perspectives on the extent to which the review’s themes were consistent with their experiences. We also consulted the group with regards to what they thought were meaningful and important recommendations for sibling interventions and supports drawing from and extending the findings of our mixed method review and their lived-in experiences. Consultation was conducted with a Sibling Advisory Panel, with members recruited as part of a larger program of sibling research. Two authors (BW, EG) held a 90-min Zoom semi-structured interview with a group of eight siblings (mean age 16.7 years, age range 9–28 years, four male and four female, two accompanied by their mothers, all Caucasian). Disabled siblings were aged 11 to 26 years, with diagnoses including autism, Down syndrome, cerebral palsy and other neurodevelopmental conditions with genetic origin (i.e. CDKL5 deficiency disorder, Rett syndrome and Trisomy 12p syndrome). These siblings consulted on the language choice in the paper (person or identity first), the ecological validity and relevance of the review’s main findings with regards to their lived-in experiences and were invited to provide their thoughts on recommendations for future sibling interventions and supports and for sibling intervention research.

A total of 1893 studies were identified from databases, 108 from registers and 24 from forwards or backwards citation searching (total 2025), of which 1101 were subject to title and abstract screening and 139 to full-text screening (see PRISMA flowchart in Fig. 1). A total of 24 studies were included: 16 quantitative, three mixed methods and five qualitative studies.

Characteristics of participants and interventions are presented in Table 2 (Appendix). Most studies were conducted in the United Kingdom (k = 7) and the United States (k = 8). Only two lower-income populations were included (Fjermestad et al., 2020 in Cambodia; Phillips, 1999 in the United States for low-income African-American families). Only three studies reported a priori power analyses (Jones et al., 2020; Kang et al., 2021; Zucker et al., 2021) and they were all underpowered to detect moderate effect size differences.

Across the 24 studies, there were 915 sibling participants (67% female) in intervention groups with sample sizes ranging from three (McCullough & Simon, 2011) to 99 (Haukeland et al., 2020), with a mean of 37. The sibling ages ranged from 3.5 years (Jones et al., 2020) to 22 years (Calio & Higgins-D’Allessandro, 2021), with a mean of 14.02 years.

The siblings with NDCs ranged in age from one to 26 years (where reported, mean 13.34 years; 77% male). Seven studies did not provide characteristics of the disabled sibling other than their NDC. Ten studies focussed exclusively on siblings of autistic children, while NDCs in other studies included ADHD, Down syndrome, cerebral palsy, intellectual disability and combined impairments (i.e. rare neurodevelopmental conditions with or without intellectual and physical disability). Only three studies reported the sibling diagnosis was clinically confirmed (Fjermestad et al., 2019; Lobato & Kao, 2002; Williams et al., 2003), while the remainder were parent report or method was not stated.

None of the qualitative or mixed methods studies included a comparison group. Nine quantitative studies (39%) included a comparison group and seven of these were randomised controlled trials (RCTs). In these nine studies, sibling participants were 4–19 years old (mean 10.5 years; 64% female). Five of the nine studies employed a waitlist-control design; two studies from the same RCT (Jones et al., 2020; Zucker et al., 2021) used an ‘attention-only’ (educational intervention focussing on general adaptive coping skills but not NDCs) participant-blinded controlled intervention; and the remaining two control groups were allocated activities not specific to being a sibling (a free-play outdoor socialisation condition; Kang et al., 2021; other psycho-educational activities; McLinden et al., 1991).

Eighteen studies included psychosocial and educational intervention components, with the most common ones being improving knowledge of NDCs and learning adaptive coping skills. Seven studies provided a parallel parent−education program to improve family cohesion and/or knowledge of NDCs. All psychosocial interventions were in group format except one, which ran 1:1 manualised support for school students (Hayden et al., 2019). Two studies ran an intervention in a school setting: after-school group format (Phillips, 1999) or individualised during school (Hayden et al., 2019). Two studies were peer-led support groups (Calio & Higgins-D’Alessandro, 2021; Naylor & Prescott, 2004). One study was run entirely online via the SibChat program (Fell et al., 2022). The most common frequency/duration was one session weekly for 6–8 weeks (k = 11), while two studies evaluated a single weekend residential programme and another a 5-day camp (Williams et al., 2003). Most studies employed a pre–post-intervention outcome evaluation design; only seven studies had longer follow-up periods of up to 6 months. Seven studies had a parent−education or participation component (D’Arcy et al., 2005; Fjermestad et al., 2020; Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Lobato & Kao, 2002; Roberts et al., 2015; Williams et al., 2003). Fifteen of the 24 studies (62.5% of studies) reported retention rates, ranging from 37 to 89%. Twelve studies (50%) provided reasons for participant attrition, including accessibility, affordability, limited interest and parents being unable to bring their children to the sessions. Only three studies (13%) reported group facilitators’ program fidelity, ranging from 71 to 100%.

Outcomes were thematically condensed into mental health/wellbeing outcomes, including anxiety (k = 4), depression (k = 6), stress (k = 4), self-esteem (k = 10), quality of life (k = 4), emotional adjustment (k = 6), social wellbeing (k = 10), family wellbeing (k = 8), coping (k = 11) and knowledge of NDCs (k = 10).

Individual study results are presented in Table 3.

Seventeen of the 24 included studies (70.83%) reported post-intervention improvements in at least one outcome. Five out of the 24 studies (20.83%) did not find any statistically significant post-intervention improvement in any outcomes, although some non-statistically significant improvements on coping skills, quality of life or social wellbeing were noted (D’Arcy et al., 2005; Fjermestad et al., 2020; McLinden et al., 1991; Naylor & Prescott, 2004; Smith & Perry, 2005). Two quantitative studies reported negative effects on at least one sibling outcome: Gettings et al. (2015) reported worse quality of life pre–post-intervention for five out of six siblings in the intervention group, while Jones et al. (2020) reported increased anxiety in both intervention and comparison groups pre–post-intervention.

Five out of the nine studies with comparison groups reported the intervention group made larger improvements compared to the comparison or waitlist groups which showed minimal changes. The other four studies using a non-waitlist comparison had mixed results: the intervention group improved non-significantly (Jones et al., 2020) and significantly (Kang et al., 2021; Zucker et al., 2021) compared to the control group post-intervention; McLinden et al. (1991) found the intervention group improved more than the comparison group on social support and positive affect, while the comparison group improved on self-esteem. Overall, therefore, all nine studies reported small to large benefits in favour of the intervention group in at least one outcome measure.

Of the four studies measuring post-intervention outcomes for more than three months, one reported significant sustained improvements for the intervention group up to 12 months’ post-intervention on self-esteem, mood, negative adjustment and behaviour problems, while the comparison groups did not maintain significant improvements at 9 or 12 months (Williams et al., 2003). Two other studies reported that the intervention group did not maintain improvement at 3 months for self-esteem or coping skills (Lobato & Kao, 2002; Roberts et al., 2015) and scores regressed to baseline pre-intervention in both studies. Finally, one study without a comparison found significantly improved sibling emotional adjustment, communication and knowledge of disability which was maintained at 4- and 7-months follow-up (Haukeland et al., 2020).

Overall, the most consistently positive sibling self-report outcomes in this review derived from the five studies which included a comparison group and follow-up data. These studies found larger positive effects on self-esteem, family wellbeing and knowledge of disability post-intervention and consistently indicated that siblings in the intervention groups appeared to benefit more than those in the comparison group (Giallo & Gavidia-Payne, 2008; Phillips, 1999; Williams et al., 2003). Siblings in interventions with combined psychosocial and educational components appeared to have better outcomes post-intervention than those with only social activities (Brouzos et al., 2017; Haukeland et al., 2020; Williams et al., 2003; Zucker et al., 2021), as were siblings in programs with parallel parenting education (compared to studies without parental involvement; Lobato & Kao, 2002; Roberts et al., 2015).

The MMAT quality assessment (see Table 4) indicated that most included intervention studies were moderate or low quality (i.e. high risk of bias), with quality scores ranging from 20 to 80%. Of the qualitative studies, Fjermestad et al. (2019) and Calio and Higgins-D’Allessandro (2021) were rated as high quality (80%). Four of the five quantitative RCTs were rated moderate quality (60%; medium level of bias). Only seven of the 24 studies described the group facilitators and their qualifications in detail, hence an assessment of clinician bias was not possible. Generalisability was considered low across studies, as most siblings were Caucasian, from high income countries and with parents who were engaged with the intervention.

The synthesis of qualitative and quantitative data resulted in four analytical themes encompassing the following outcomes: emotional and behavioural adjustment (k = 14, including anxiety, depression and emotional adjustment), knowledge and education (k = 14, including NDC knowledge and coping), inter-personal wellbeing (k = 14, including family and social wellbeing) and intra-personal wellbeing (k = 19, including self-esteem, stress and quality of life) (Table 5).

Our review synthesis showed that the strongest positive improvements for siblings across all included intervention studies were noted for increasing knowledge of NDCs (k = 10) and improving self-esteem post-intervention (k = 8). The sibling wellbeing domains that appeared to show fewer improvements post-interventions were anxiety (k = 2 improved, 2 little/no change, 1 decline), depression (k = 3 improved, 2 no change) and quality of life (k = 2 improved, 1 no change, 1 decline).

To contextualise the self-report outcome findings presented in this review, we briefly explore in this section how these compare to the more commonly studied and reported parent-reported outcomes in the studies which included these. Ten studies included one or more parent−report measures of sibling emotional and behavioural functioning, with parents consistently reporting larger sibling improvements in mental health post-intervention as compared to sibling self-report. Five of the ten studies used the parent-reported SDQ, all finding improvements post-intervention and significant group differences favouring the intervention group (Fjermestad et al., 2020; Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Hayden et al., 2019; Roberts et al., 2015). Of these five studies, three also measured sibling self-report SDQ and contrastingly to the parents, the siblings either did not report significant improvements (Fjermestad et al., 2020; Roberts et al., 2015) or reported similar improvements to paternal but not maternal report in one study (Haukeland et al., 2020).

Eighteen of the 24 (75%) studies examined the siblings’ overall satisfaction with the intervention. All 18 evaluations were positive, with siblings reporting enjoyment, improved knowledge of NDCs, meaningfulness or personal development. In six out of the 24 included studies, a minority of siblings also provided some negative feedback/evaluation points, in which other participant’ behaviours (interrupting, pressuring to speak; Fjermestad et al., 2020) were noted and one sibling suggested the need for greater ethnic diversity (Fell et al., 2022).

Siblings with lower baseline self-reported resilience-promoting factors (such as fewer coping skills and lower self-esteem) tended to show the most improvements post-intervention in the 11 quantitative and three qualitative studies exploring these (Jones et al., 2020; Smith & Perry, 2005). Three studies with siblings with baseline parent-reported ratings on emotional and behavioural adjustment in the normative range also found significant, yet smaller, pre–post-intervention improvements. The largest positive intervention results tended to include siblings of a younger age (mean age of less than 11 years) and samples with more male than female participants (Brouzos et al., 2017; Kryzak et al., 2014). However, these patterns were contradicted in some studies: a study with all females reported positive changes in psychosocial adjustment (Rye et al., 2018); a study of siblings aged 20–22 reported significant improvements in wellbeing (Calio & Higgins-D’Allessandro, 2021), while adolescents (aged above 12) improved more on self-esteem than children (Williams et al., 2003).

Regarding the type of NDC, the studies with larger positive effects tended to include siblings of individuals from a heterogeneous group of NDCs compared to studies with siblings of individuals with one NDC such as autism (Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Phillips, 1999; Williams et al., 2003). However, this difference may be due to the severity of the NDC or behavioural problems of the child with the NDC, rather than syndrome/condition specificity (Jones et al., 2020). Additionally, these studies have different intervention components and measures, which may result in a trend towards more positive results.

Confidence in the body of evidence per outcome is presented in Table 6. The most certainty in the overall evidence was rated high for self-esteem, social wellbeing, family wellbeing and NDC knowledge. The quality of life outcome was rated very low due to small sample sizes, non-standardised or heterogeneous instruments used and inconsistent results.

The Sibling Advisory Panel were presented with the key review findings in a lay presentation and were invited to share their thoughts and feedback. Sibling community participants agreed with the presented outcomes. Most siblings had previously participated in a family intervention targeting their disabled sibling, which they had found beneficial for improving family communication.

There were two main recommendations. First, siblings shared that at school they experienced bullying, segregation, social isolation, stigma and the staff were under-skilled to support siblings’ unique needs or educate other students on disabilities. One sibling stated intervention targets in schools could include “compassion and kindness, improved self-esteem, improved social relationships with friends, less feelings of shame and greater support within the school environment”. Therefore, a school-based psycho-educational intervention for peers and staff may be beneficial to foster acceptance and understanding. A second recommendation was regarding the burden of responsibility and increasing anxiety with age; siblings indicated there should be peer-led support groups and education on future planning offered to older siblings.

The current review of 24 intervention studies identified ten sibling mental health/wellbeing outcomes studied in intervention research to date: anxiety, depression, stress, self-esteem, quality of life, emotional adjustment, social wellbeing, family wellbeing, coping and NDC knowledge. Four analytical themes encompassing these outcomes derived from both quantitative and qualitative results and the sibling community involvement included (i) emotional and behavioural adjustment, (ii) education and knowledge, (iii) inter-personal wellbeing and (iv) intra-personal wellbeing.

The largest immediate post-intervention improvements were in self-esteem, social wellbeing and knowledge of NDCs. The most sustained improvements at follow-up periods in the few studies that explored these were in emotional and behavioural adjustment and in NDC knowledge. The outcomes with less consistent findings and smaller post-intervention or group differences were anxiety, depression and quality of life, although this may also have been due to floor effects or selection bias amongst sibling participants. The intervention groups generally had better outcomes than the waitlist control and active comparison groups.

In general, qualitative studies reported more positive intervention effects than quantitative studies on sibling wellbeing outcomes (Rye et al., 2018) It may be that standardised measures used in current intervention studies are not capturing outcomes considered important to siblings (Calio & Higgins-D’Allessandro, 2021). It is also possible that siblings may be providing socially desirable answers in the interviews, although three studies had anonymous feedback forms and in interviews asking for both positives and negatives of the intervention, the positives outweighed the negatives (Evans et al., 2001; Fjermestad et al., 2020; Giallo & Gavidia-Payne, 2008).

Qualitative results from the sibling stakeholder consultation resulted in two additional recommendations: (i) interventions within the school context are needed to improve acceptance, compassion and understanding and (ii) education or support programs exploring and supporting future planning responsibilities targeting older siblings to reduce anxiety and worries about the future. Overall, siblings reported benefitting from interventions and it may be that mixed methods of data collection or different instruments are needed to capture the spectrum of sibling experiences.

This research question remained largely unanswered due to non-reporting or large heterogeneity of demographic information/sample characteristics and few studies reporting pre-intervention cognitive, emotional or behavioural functioning. Trends in included studies indicated that sibling participants of younger ages (D’Arcy et al., 2005; Hayden et al., 2019; Zucker et al., 2021), male siblings (Brouzos et al., 2017; Lobato & Kao, 2002), siblings of individuals with lower disability symptom severity (Jones et al., 2020) and siblings with poorer baseline (clinically elevated) functioning as reported by parents (Giallo & Gavidia-Payne, 2008; Roberts et al., 2015), tended to report more improvements from the interventions or support groups. However, poorer baseline functioning may have caused ‘ceiling effects’ (Giallo & Gavidia-Payne, 2008; McLinden et al., 1991). No quantitative studies adopted a strengths-based approach by measuring positive impacts of having a sibling with a disability or baseline individual-level resilience-promoting factors (e.g. trait resilience, optimism, emotion regulation) which may be predictors of benefit from intervention (Green, 2013). This is problematic, as support groups may be targeting siblings who may not need or benefit from targeted interventions and may be missing key risk groups of siblings (Tudor & Lerner, 2015).

Interventions in group format held weekly for 6–8 weeks, with 60–90 min sessions, appeared to have more positive outcomes (Brouzos et al., 2017; Kryzak et al., 2014; Lobato & Kao, 2002; Roberts et al., 2015). However, there was minimal consensus in the approach adopted in interventions, even for those using similar manualised programs, although it is apparent that group designs were most widely used (21 of 24 studies) and produced some positive outcomes for siblings (D’Arcy et al., 2005; Kryzak et al., 2014; Lobato & Kao, 2002; Roberts et al., 2015). Due to this heterogeneity, it is difficult to compare the influences of different types or approaches to interventions on sibling outcomes (McKenzie Smith et al., 2018). As also commented in previous reviews, no psychosocial intervention included in this review qualifies as ‘well-established’ or ‘probably efficacious’ (Tudor & Lerner, 2015) and none of the intervention studies included were a ‘high quality’ randomised controlled trial (Higgins & Green, 2011). However, a key intervention component common in several of the studies reporting some improvements/positive outcomes was parental involvement. Seven studies used a parallel parent education component with all but one (Fjermestad et al., 2020) reporting significant improvements in outcomes including NDC knowledge, self-esteem, family communication, positive attitudes and parent-reported behaviour problems (D’Arcy et al., 2005; Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Lobato & Kao, 2002; Roberts et al., 2015; Williams et al., 2003). In two of these seven studies which included parent reflections on the interventions (Giallo & Gavidia-Payne, 2008; Roberts et al., 2015), parents shared that they were often not aware of the negative impact of the NDC on the non-disabled sibling and described improved parenting skills and family communication post-intervention (Fjermestad et al., 2019; Gettings et al., 2015; Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Lobato & Kao, 2002). Collectively, this evidence suggests the involvement of parents in sibling intervention programs and parallel parent education programs, may be an important ingredient. The discrepancy between parent and sibling reported outcomes was evident in seven of the included studies that assessed both, with parents generally reporting larger sibling improvements in mental health post-intervention compared to sibling self-report (Fjermestad et al., 2020; Giallo & Gavidia-Payne, 2008; Haukeland et al., 2020; Hayden et al., 2019; Roberts et al., 2015). These highlights the crucial role of self-reported experiences being central in future intervention studies’ primary outcomes and questions the sensitivity of parents as sole informants (Lobato & Kao, 2002).

Our review included 13 studies not included in the five existing reviews (Hartling et al., 2014; Kirchhofer et al., 2022; McKenzie Smith et al., 2018; Thomas et al., 2016; Tudor & Lerner, 2015). Similar to the previous systematic reviews, our study found overall positive effects of interventions in over 65% of the included studies, primarily for self-esteem, social support and knowledge of NDCs. In contrast to all five earlier reviews, however, the current review found little evidence for sibling self-reported improvements in anxiety, depression or quality of life. The finding in this review that parents consistently reported greater sibling emotional and behavioural improvements following interventions as compared to siblings’ self-reports is difficult to interpret, as all earlier reviews combined parent and sibling reports and the heterogeneity across studies makes comparisons difficult. Our focus on sibling self-report; therefore, revealed unique sibling needs and experiences not explored in previous studies or reviews. In addition, the qualitative components of studies included in this review, in addition to our community consultation, identified new sub-themes from the data: specifically, we found evidence for improved intra-personal wellbeing including enhanced resilience and self-worth (Calio & Higgins-D’Allessandro, 2021; Rye et al., 2018) not evident in previous reviews evaluating quantitative outcomes.

The primary limitation across studies was under- or non-reporting of key variables; few studies reported family demographics and many included heterogeneous participant characteristics. The use of non-standardised or heterogeneous instruments may have contributed to contradictory results, such as reduced quality of life found in Gettings et al. (2015). Most studies only reported mean scores rather than using measures which could report the proportion of siblings in borderline or clinically elevated range pre- and post-intervention, which would have provided more insight into baseline characteristics of siblings benefitting most from interventions (Giallo & Gavidia-Payne, 2008; Ma et al., 2017). All included studies only assessed one sibling in the family, commonly the sibling closest in age to the target child, which may introduce sampling bias (Ma et al., 2017).

As many studies used a within-subjects pre–post design with small samples and without a comparison group (Kryzak et al., 2014; Lobato & Kao, 2002; Smith & Perry, 2005), no assumptions about superiority of sibling interventions over other forms of support or services which may also have positive effects can be made (Fjermestad et al., 2020). Additionally, using comparison groups of siblings receiving individualised or specific components of interventions would enable mechanisms of change and intervention components to be identified (Roberts et al., 2015). The small sample sizes and non-reporting of data did not allow analysis of sub-groups to identify any moderator effects in this review. There are likely groups of higher-risk siblings experiencing a range of individual, family or broader sociocultural/structural risk factors, who may benefit more from personalised/individual interventions and a broader range of supports and services.

Replicability of the intervention design would be problematic, as most studies did not report key components of the interventions or interview protocols. There was a lack of empirical evidence behind intervention design, other than the use of psychoeducation. In the current review, most studies did not include a follow-up and those which did had high attrition (Haukeland et al., 2020). Clinical significance, not simply statistical significance, should be assessed in more studies; only one study evaluated clinically meaningful change in individual siblings (Giallo & Gavidia-Payne, 2008).

The current review has limited generalisability due to heterogeneity of reported family demographics, small sample sizes and non-reporting of data which did not allow this review to conduct meta-analyses or quantitative moderation analyses of sub-groups. There are likely groups of higher-risk siblings who may benefit more from psychosocial interventions (McKenzie Smith et al., 2018; Tudor & Lerner, 2015). For instance, the type and severity of NDC of the disabled child likely has differential impacts on siblings (Mandleco & Webb, 2015). In the current review, included studies emphasised that having an autistic sibling may present unique challenges distinct from other NDCs (Zucker et al., 2021), consistent with prior literature (Kaminsky & Dewey, 2002; Orsmond & Seltzer, 2007). In the sibling community consultation, all participant siblings were Caucasian, middle to upper socioeconomic status and with educated parents; most were older than their diagnosed sibling and most siblings with NDCs were autistic, hence the sibling community consultation group is not representative of all siblings of people with NDCs.

Methodologically, the primary future direction for future intervention/support studies is greater replicability and stronger evidence drawn from larger sample sizes for sibling reported outcomes. Replication studies evaluating the same type of intervention, with the same study population and outcome measures with repeated assessments are needed to enable comparisons across studies (Tudor & Lerner, 2015). The use of active comparison and waitlist control groups with a larger sample size in all intervention studies will further improve the clinical utility of controlled trials (Hartling et al., 2014). Studies should further explore and report other possible moderating variables or demographics known to impact sibling psychological functioning and appropriate power analyses should be conducted to allow testing of such research questions. There is also a need for a more collaborative approach across research teams and community, clinical or school-based supports.

Secondly, in addition to tailoring interventions based on sibling/family context and accessibility, the evidence from this review and the sibling community consultation suggests tailored interventions may be beneficial for siblings at different life periods. For instance, younger children tended to show greater improvements on knowledge of NDC (Brouzos et al., 2017), whereas adolescents improved more on self-perception and self-esteem (Williams et al., 2003). The oldest participant across identified studies was aged 22, indicating a lack of intervention studies for older siblings in emerging adulthood. Older siblings have increased anxiety regarding caregiving responsibilities as their disabled sibling ages (Macks & Reeve, 2007; Stampoltzis et al., 2014) and they benefit from support programs targeting sibling empowerment and community connection (Burke et al., 2020). Other stages are important to consider, such as immediately after the initial diagnosis of the disabled sibling and at key developmental transition periods (Hastings, 2016; Petalas et al., 2009). Other family characteristics or dynamics (such as family cohesion, family stressors, family structure, size and resources; see section “Introduction”) may be moderating variables impacting mental health outcomes and although these were not explored or targeted in the included studies, it is important that future multi-targeted support programmes are developed and systematically evaluated, over and above programmes targeting solely the siblings.

Finally, accessibility of interventions across siblings from all socioeconomic and ethnic backgrounds should be considered. Further, our results indicated that in six studies, siblings were unable to access services due to parental time or financial constraints (Giallo & Gavidia-Payne, 2008); previous reviews also emphasise problems with reliance on clinic-based services with waiting lists, family financial strain or fear of causing additional stress for parents (Green, 2013). This review found that only two studies targeted non-Caucasian ethnicities or low-income families and these studies suggested that these families may have unique negative experiences due to disadvantage, stigma or cultural expectations (Fjermestad et al., 2019; Phillips, 1999). As only one study occurred in a school context (Hayden et al., 2019), further research is required into programs delivered in education settings and via more accessible service providers.