--- - |2+ Emerging evidence suggests that sibling aggression is associated with the development of high‐risk behavior. This study investigated the relationship between sibling bullying perpetration and victimization in early adolescence and high‐risk behavior in early adulthood. Sibling bullying was assessed at 12 years in 6,988 individuals from the Avon Longitudinal Study of Parents and Children, a birth cohort based in the UK and high‐risk behavioral outcomes were assessed at 18–20 years. Frequent sibling bullying perpetration predicted antisocial behavior (OR = 1.74; 95%CI, 1.38–2.20), while frequent sibling bullying victimization increased the odds of nicotine dependence (OR = 2.87; 95%CI, 1.55–5.29), even after accounting for peer bullying and parent maltreatment. Categorical analysis revealed that particularly bullies and bully‐victims were at risk of developing high‐risk behavior. Finally, this study found that adolescents who were involved in bullying perpetration across multiple contexts (home and school) had the highest odds of reporting antisocial behavior (OR = 3.05; 95%CI, 2.09–4.44), criminal involvement (OR = 2.12; 95%CI, 1.23–3.66), and illicit drug use (OR = 2.11; 95%CI, 1.44–3.08). Findings from this study suggest that sibling bullying perpetration may be a marker of or a contributory factor along the developmental trajectory to antisocial behavior problems. Intervention studies are needed in order to test whether reducing sibling bullying can alleviate long‐term adverse social and behavioral outcomes. - Aggressive Behavior, EarlyView.