Among typically developing children, having sibling relationships promotes the development of social skills. This is a retrospective study of the effect of having sibling/s on the severity of the clinical presentation of autism spectrum disorder (ASD). The study included 112 children, 99 males and 15 females, mean age 29.6 ± 9.2 months, diagnosed with ASD. The study population was composed of a group of children with ASD who had older typically developing sibling/s (n = 56) pair-matched for age and cognitive level to a group of children with ASD without sibling/s. Each participant underwent a comprehensive assessment using standardized tests. The group with older sibling/s had less severe observed social deficits (Autism Diagnostic Observation Schedule-Social Affect calibrated severity scales [ADOS-SA-CSS]) and fewer reported non-verbal communication impairments (Autism Diagnostic Interview-Revised [ADI-R]). Regression analyses revealed that, for the ADOS-SA-CSS, higher cognitive level and having older sibling/s were associated with less severe observed social affect deficits. This model explained 32.0 % of the variance. For the ADI-R communication scores, older age, higher cognitive level and having older sibling/s were associated with less severe reported non-verbal communication impairments. This model explained 33.0 % of the variance. The main finding in this study is that a familial factor, specifically having older sibling/s, was associated with better social communication abilities in children with ASD, in addition to age and cognitive ability. Having sibling/s may offer opportunities for the child with ASD to experience social interactions with children and to acquire communication skills.