The longitudinal associations of older sibling substance use as well as dyadic sibling conflict and collusion to younger sibling substance use were examined in a community-based sample of 244 same-sex sibling pairs. Indirect effects of older siblings on younger sibling substance use were hypothesized via younger sibling deviant peer affiliation and conflict with friends. Adolescents, parents, friends, and teachers completed measures of substance use, conflict, and deviant peer involvement. Observational data were used for both measures of collusion and conflict. Findings suggest that older sibling substance use has a direct effect on younger sibling use, but relationship dynamics and reinforcement played a significant role as well. Specifically, collusion and conflict in the sibling relationship both had indirect effects through younger siblings' deviant peer affiliation. Findings validate the powerful socializing role of both siblings and peers, and elucidate the complex mechanisms through which socialization occurs. Furthermore, data underscore the importance of considering how multiple dimensions of socialization operate in the elaboration of antisocial behavior.