Family environment emerged as a significant factor associated with internalizing problems and overt/relational aggression for both younger and older siblings. An important family factor was sibling perceptions of maternal psychological control. Psychologically controlling mothers attempt to shape children's behaviors through tactics such as shaming, instilling persistent anxiety, and using conditional approval. Younger siblings' perceptions of maternal psychological control were associated with their own overt/relational aggression and internalizing problems, and older siblings' perceptions of maternal psychological control were associated with their relational aggression only. Differential maternal treatment between sibling pairs was associated with higher levels of internalizing problems among younger children, but was not significantly related to higher levels of internalizing problems among the older siblings, except in the case of older males with younger male siblings. Although causality cannot be determined with the current data, there is mounting evidence that suggests both relationally and overtly aggressive behavior is substantially stable within individuals over time and is predictive of later internalizing problems. Positive family environments characterized by warmth, expression of feelings, and supportive guidance resulted in less aggressiveness and positive adjustments in sibling relationships and overall adjustment in adolescence. A Web site survey measured positive parental expressiveness, family cohesion, maternal differential treatment of members of sibling pairs, maternal psychological control, sibling overt and relational aggression, and sibling internalizing problems. 3 tables, 2 figures, and 121 references