The prospective associations of mothers’ coercive and authoritative parenting processes and youth disclosure with youth delinquent behavior (externalizing behavior problems and delinquent peer affiliation) were examined in a longitudinal design following 244 sibling dyads (122 brother pairs/122 sister pairs) from mean ages 10 and 13 years (younger and older siblings, respectively) to mean ages 13 and 16 years. Questionnaire data from multiple informants were used to assess the role of youth disclosure in mediating the prospective association of maternal parenting processes with youth delinquent behavior, contrasting younger sibling models with and without older sibling influence at age 13 years. For both older and younger siblings, authoritative parenting was positively associated with youth disclosure and negatively related to delinquent behavior, and coercive parenting was negatively associated with youth disclosure and positively related to delinquent behavior. However, youth disclosure did not mediate the relationship between parenting processes and youth delinquent behavior. When the influence of older sibling delinquency on youth sibling delinquent behavior was added to the model, younger sibling disclosure fully mediated the relationship of parenting processes to subsequent delinquent behavior. The data indicate the important role of sibling influence in the development of delinquent behavior at the transition to adolescence, powerfully contextualizing the relative roles of parenting processes and youth disclosure in the transmission of risk. The relative contribution of youth disclosure to delinquent behavior appears to be influenced by parenting processes and by developmental timing, and to be conditional on sibling influence.