Research on sibling aggression remains limited despite growing evidence that aggression towards siblings is associated with negative adjustment outcomes. The current study advances the field by examining proactive and reactive aggression toward siblings in middle childhood. Specifically, this study utilized youth self-reports to 1) evaluate the 6-month stability of sibling proactive and reactive aggression, 2) compare rates of sibling aggression to peer aggression, and 3) evaluate links between sibling proactive and reactive aggression and internalizing symptoms (depression and anxiety). Findings indicated stability in both sibling proactive and reactive aggression, and rates of sibling aggression were higher than rates of peer aggression at both time points. Further, as anticipated, both sibling and peer reactive, but not proactive, aggression were uniquely positively associated with internalizing symptoms. Results suggest that sibling proactive and reactive aggression can be reliably assessed and are associated with expected outcomes, but peer aggression demonstrated stronger clinical utility than sibling aggression. Future directions are discussed.