What do you do if your mom is cheating on your father?

Cheating Mom! Of course, I would have liked to know how old you are! But, if you are a teenager, I would suggest that you talk to your mother. Say for example that "I know that you are seeing someone, mother" - be prepaired to tell her the details about how you came to know - "and it hurts me." You may continue. "I love dad! But I'm not going to tell him. Still, mother, I would prefer if you would not put me in the middle of this thing." This is about the best you can do. Hopefully your mother will listen. But should she not, if I were you, I would stay out of this situation. Adults have odd and surprising ways of getting you involved in their affairs. Hopefully, in your adult life, you'll find someone with whom you can establish a better relationship.