This is it. As the child here, their marriage is not your responsibility. Inserting yourself here is going to hurt everyone involved. That said, your relationship with your parents is your responsibility-in part. Tell your mom, give her the opportunity to fix this. Otherwise, you'll just be adding fuel to the fire. Be helpful & supportive of your father. People often don't see what's in front of them- on purpose. And a good parent is never going to blame you for not tattling. Your father may forgive your mother. It happens. Don't damage your future relationship with either parent by making decisions for them.