Children often learn and apply behaviors in sibling interactions before they routinely use such behaviors in peer interactions. As part of a longitudinal study of relational aggression, 46 children were videotaped at home in semi-structured free play situations with siblings and with friends at age 4 and again at age 8. Observations revealed that sibling dyads exhibited a higher rate of relationally aggressive behaviors than did friend dyads at age 4, but by age 8, sibling and friend dyads showed similar rates of relationally aggressive behavior. Overall, this shift occurred because relational aggression between siblings decreased whereas relational aggression between friends increased. However, this pattern varied to some degree with the sex of the target child and whether the sibling was older or younger than the target. These results suggest significant implications for the development and transmission of children's social behaviors across contexts, highlighting the importance of intervening early in childhood and in the sibling context.