If it has happend more then once I would get her help and fast. Make sure u tell her it isn't her fault and that u aren't mad at her. As for the boy, talk to his parents, there is no telling who else he has touched. He could be a victim or just a typical horny (excuse my language) boy. Either way it is unaceptable. He too needs help! Whoever was with them, should have been paying more attention. Kids are sneaky. I wouldn't trust them to be alone anymore, for ur daughters sake I say keep her away from him as much as possible.
When you talk to her tell her if is ever happens again with anyone,she must tell you immediatly,I would keep her away from her half brother and speak to his parents he may do this to other children .so you have to deal with it ...Good Luck..