It is a refreshing change of pace to hear the child's perspective of her parents' marriage falling apart. As a mother, I tend to hide a lot of adult issues from my kids. It is very unfortunate that you got caught in the middle. How old is your sister? If you are both adults, can't you just move out? There is bound to be more drama about to occur. Your dad will eventually find out and not tolerate it, and the other guy will also need to tell his wife and kids and everything will fall apart. Then maybe pieces can be picked up, and that's the only way things will resolve. I hope everyone is emotionally strong enough to handle it.