We adopted a second child when our first adopted son was 2. He had several adopted friends in the neighborhood. I thought I was so wise to let him decide when he wanted to introduce his sister to his friends. After about a month he decided it was time. He brought his friends into the kitchen where his sister was lying in a bassinet. He walked up to the bassinet, lifted the blanket near the bottom, picked up his sister's foot and bit it. He had not shown any aggression up to that time.
Postscript: They are not close at all, and he was diagnosed with Antisocial Personality Disorder.