Research indicates that older siblings uniquely influence their younger brothers' and sisters' substance use behaviors during adolescence; however, the underlying mechanisms of socialization are rarely examined. The present study investigated whether social and/or cognitive pathways mediated the association between adolescent siblings' alcohol use and whether these pathways were moderated by the gender composition of the sibling dyad. Participants included one parent and two adolescent siblings (M age = 14.52 and 17.17 years) from 326 families. Data were collected via telephone interviews. Path analysis demonstrated that the association between older and younger siblings' alcohol use was mediated via social and cognitive pathways. Specifically, older siblings' drinking was positively related to the frequency of siblings' co-use as well as more positive expectations about alcohol, which in turn were positively associated with younger siblings' alcohol use. Identifying the ways in which siblings influence each other's substance use and health is critical because they are emerging and effective targets of intervention and prevention.