The study explored whether sibling affection and hostility were longitudinally associated with adolescents' prosocial, externalizing, and depressive behaviors, after controlling for parent–child and best friend relationship quality. Sympathy was examined as a possible mediator. Three hundred and eight randomly selected families completed Waves 3, 4, and 5 of the Flourishing Families Project. Multiple group comparison via structural equation modeling compared differences between girls and boys. Sibling affection (T3) was positively associated with adolescents' sympathy (T4) and prosocial behavior (T5). Sibling hostility (T3) was positively associated with adolescents' depression (T5) and externalizing behavior (T5) (for boys only), even after controlling for parent and friend relationships. Discussion focuses on the unique role of the sibling relationship on adolescent development.