Prospective associations of mothers’ parenting processes, youth disclosure, and youth problem behavior were examined in a longitudinal design following 244 adolescent sibling dyads over a 3-year period. For both siblings, authoritative parenting was positively associated with youth disclosure and was negatively related to problem behavior, and coercive parenting was negatively associated with youth disclosure and was positively related to problem behavior. When the influence of older sibling problem behavior on younger sibling problem behavior was modeled, younger sibling disclosure accounted for the relationship of maternal parenting processes to problem behavior. Findings indicate the important role of sibling influence in the development of problem behavior, contextualizing the relative roles of maternal parenting and youth disclosure in the transmission of risk.