Background: Siblings may support each other, but also reveal fierce rivalry and mutual aggression. Supportive sibling relationships have been linked to the development of psychosocial competence of children. In the present longitudinal study, we will focus on the development of perceived support in sibling dyads and on the influence of sibling support and sibling problem behavior on psychosocial adjustment in adolescence. Method: In a three-wave longitudinal sample of 285 Dutch families with two adolescent children (11- to 15-year-olds), these two siblings judged the support perceived from each other. In addition, they themselves and their parents judged their internalizing and externalizing problem behaviors. The relation of sibling support and sibling problem behavior with internalizing and externalizing problem behaviors was examined while controlling for support from parents and friends and, over time, controlling for the autoregressive effects of problem behavior. Results: Support perceived from a sibling is mostly negatively related to externalizing problems; sibling problem behavior is strongly related to internalizing problems. Differential developmental trajectories of adolescents' adjustment are associated with siblings' support and problem behavior. Conclusion: The results indicate that adolescents' relationships with both older and younger siblings are characterized by modeling processes.