Studied here were the links between sibling differences in trajectories of change in the qualities of parent–child relationships and the qualities of sibling relationships across a 2-year period in adolescence. Participants were first- and second-born siblings (M age=14.94 years for firstborns and M age=12.46 years for secondborns) from 185 predominantly White, working and middle-class families. In home interviews, siblings reported on their dyadic family relationships. For reports of parent–child warmth but not parent–child conflict, results were consistent with sibling differentiation theory: Increasing differences between siblings over time in parent–child warmth were linked to trajectories of increasing warmth and decreasing conflict in the sibling relationship as reported by firstborns, and increasing warmth in the sibling relationship as reported by secondborns. The findings support the view that sibling differentiation may be a strategy for managing sibling conflict and rivalry.