The role of siblings ("N" = 50) in the display of physical and relational aggression among peers during early childhood was explored. Specifically, sibling pairs' rates of physical and relational aggression were assessed in their independent social contexts. Findings indicated low to moderate levels of intercorrelation between physical and relational aggression and moderate levels of stability for both physical and relational aggression across an academic year. Observations revealed that older sisters were more relationally aggressive than older brothers, whereas older brothers were more physically aggressive than older sisters. Older siblings directed more aggressive behavior to same-sex peers than did their younger siblings. Older sibling's relational aggression predicted younger sibling's use of relational aggression towards peers. In addition, older sibling's physical aggression predicted younger sibling's physical aggression with peers. This study reveals the importance of a multi-contextual approach (i.e., school and family influences) in understanding the development of aggression and in providing a guide for future interventions.