Not really sure what this tells us or what we'll do with it, but it's interesting:
In sum, the presence of both older and younger siblings influences the development of aggressive behavior in adolescence. Having a brother or a highly aggressive sibling of either gender can lead to greater increases in aggression over time.
Researchers looked at 451 sibling pairs, ages 9 through 18, and their parents. The adolescent siblings each rated their own aggressive behaviors, and parents described economic pressures on the family, such as difficulty paying bills. Trained observers assessed the hostility the parents directed toward each adolescent during family interactions. In their work, the researchers took into consideration the age difference between the siblings as well as such factors as parenting styles and family economics.
The study also found that older siblings who were aggressive tended to have younger siblings who were also aggressive, and vice versa. This association was found for sibling pairs with two boys, two girls, and one boy and one girl. Aggression in younger siblings also predicted increases in aggression in older siblings over time, and vice versa, though the extent varied according to each sibling's gender.
Parents' hostility also played a role in the development of aggression in their children. Family economic pressure predicted increased aggression indirectly, through its association with parental hostility.