This study examined the associations between sibling intimacy and conflict and youths' reports of risky behavior in a sample of adolescents ages 11-20. Participants were mothers, fathers, and sibling dyads in 393 families who were interviewed annually for 3, 4, or 5 years. Multivariate multilevel models tested longitudinal links between sibling intimacy and conflict and youths' risky behavior and whether these associations were moderated by birth order, sex, or dyad sex constellation. Controlling for parent-youth conflict, the results showed positive within-person covariation between sibling conflict and risky behavior for all youths except firstborns with younger brothers. Controlling for parent-youth intimacy, sibling intimacy was positively linked with risky behavior at the between-person level, but only in brother--brother pairs. The discussion focuses on sibling relationships as a context for adolescents' individual development and the roles of birth order, sex, and dyad sex constellation.