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.