AbstractThis study examined how emerging adults' perpetration of aggression toward a sibling closest in age was longitudinally associated with their sibling relationship quality. Emerging adults (N = 143; Mage = 19.62; 70% female) completed surveys online or by mail at two time points, 4 years apart. Of emerging adults, 25% perpetrated aggression against their closest‐in‐age sibling. Perpetration of sibling aggression was predictive 4 years later of less sibling warmth, involvement, and emotional help. None of the interactions with gender and sibling gender was significant. This study's findings demonstrate the importance of examining the consequences of perpetration of aggressive behavior toward a sibling for sibling relationship quality in emerging adulthood.