I understand what you feel. And reporting your dad of his offense could only worsen the situation. Making him go home with a bad record will not solve the dilemma. Just remember, your mom loves you and will appreciate if you do something for her. This is not your fault. Your parents will have to sort it between them, it's not something you should have to try and sort out. I would say keep loving your parents, in the long run she will be better off knowing as I doubt she would want to continue as normal with your dad if he is with somebody else. Best of luck!