Evaluated sibling effects of a family-based intervention aimed at preventing conduct problems in preschool-aged siblings of adjudicated youths. Ninety-two families of preschoolers who had older siblings adjudicated for delinquent acts were randomly assigned to intervention and control conditions. We hypothesized group differences on antisocial behavior and peer relations for older siblings of targeted preschoolers. We examined outcomes of parent and teacher-reported behavior at baseline (T1), immediately post-intervention (T2), and 8-months post-intervention (T3) for school-aged (5 – 11 years) and adolescent (12 – 17 years) siblings. Findings revealed significant intervention effects 8 months following intervention for adolescent siblings on parent-reported antisocial behavior and peer relations. Teacher reports confirmed group differences for antisocial behavior. Findings document benefits for adolescent siblings.