Until very recently, the role of siblings in the development of physical aggression in children has been largely ignored. However, in light of a growing body of research, siblings may be significant risk factors in such development. The purpose of the present study was to assess the unique, independent influence of older sibling's physical aggression on the development of physical aggression in younger children. A longitudinal sample of 1,441 target children, between the ages of four to 11 years across three time points, who had an older sibling was employed. Growth curve modeling was used to identify patterns of change over time (within-individual differences) as well as factors that may account for differences between children with respect to this pattern of change (between-individual differences). It was found that for the average target child, a one year increase in age was associated with a significant decline in rates of physical aggression. Significant differences were revealed between boys and girls, with boys demonstrating higher levels of physical aggression and a steeper decline over time than girls. With regard to between-individual differences, witnessing violence within the home, maternal depression, socioeconomic status, and single parenting were significant predictors of levels of physical aggression for target children. In addition, the presence of a physically aggressive older sibling independently predicted younger children's change over time in rates of physical aggression. Children with physically aggressive older siblings demonstrated a less rapid decline in their rates of physical aggression than did children without physically aggressive older siblings. Findings are discussed in terms of how family-based factors differentially influence the development of physical aggression in children.