You love your dad and your sister and you don't want to hurt them by betraying them to their partners, but the truth is that what they are doing is WRONG and UNFAIR and if you have the ability to stop them and your conscience tells you that you should, there's nothing else for it. You should. However, if you can't help and if what you say will make no change, imho the best thing you can do is to learn not to care, which of course is not easy :s