The contribution of younger male and female siblings’ conflict and involvement in deviant activities with their older brothers to younger siblings’ adolescent adjustment problems was examined in the context of parenting. Ineffective parenting during younger siblings’ childhood had no direct effects on adjustment, but facilitated their exposure to older brothers’ deviant peers and activities. The effect of sibling conflict on adjustment was mediated by younger siblings’ co-participation in deviant activities with their older brothers during adolescence. Early sibling conflict and co-participation in deviant activities synergistically increased risk for younger siblings’ adolescent adjustment problems. These empirical relations held in the context of parental discipline of younger siblings during adolescence. Sibling relationships entail a set of iterative social processes that strongly influence risk for adolescent antisocial behavior, drug use, sexual behavior and traumatic experience. Variations in sibling influence were observed conditional on the gender combination of the sibling pair and on.