This 3-wave, 5-year longitudinal study tested the contributions of family contextual factors and sibling relationship qualities to younger siblings' substance use, sexual risk behaviours, pregnancy, and sexually transmitted disease. More than 220 non-White families participated (67 percent Latino and 33 percent African American), all of which involved a younger sibling (133 girls and 89 boys; mean age=13.6 years at Time 1) and an older sister (mean age=17 years at Time 1). Results from structural equation latent growth curve modelling indicated that qualities of the sibling relationship (high older sister power, low warmth/closeness, and low conflict) mediated effects from several family risks (mothers' single parenting, older sisters' teen parenting, and family's receipt of aid) to younger sibling outcomes. Model results were generally stronger for sister-sister pairs than for sister-brother pairs. Findings add to theoretical models that emphasize the role of family and parenting processes in shaping sibling relationships, which, in turn, influence adolescent outcomes.