I hate it when people say because you're all minors that it's not harmful. I had a sibling--12 years older--who blew it all off because he said he was "just a kid" even though he was 16-18 at the time. Anyway, 13 years old is too old to be playing like that with an 8 year old. She may have been abused herself--doesn't excuse her behavior, but it might explain it.