I think that siblings would play an important role in developing a secure attachment style. Especially if the kids are close enough in age, they would definitely interract with one another long before they began to interract with children from outside families. Another thing is dealing with only children. Although I may be generalizing, it seems that only children are less willing to compromise and are sometimes selfish. This could be because their parents solely focused their full attention only on them. This behavior is deeply rooted in the brain, and it could be possibly more difficult for an only child to learn behaviors like sharing and compromise.
