Sorry that this is really hard for you. I would go with what your mom tells you. I wouldn't really think the issue is about the cheating so much. I would say it's because it's the young girls. What is really more disturbing is it was your sister, and her friend. Also, he is clearly saying that he has a problem if he is going to counseling. It might be hard, but this is one man I would let go. If your not so sure than I would say go to counseling with him just to find out what is going on in his mind.