You need to sit down with your parents, either all three of you together, or one on one. But you need to tell your mom this is not right, and how it's affecting you. Your her child, and she shouldn't be doing this, it will only make life harder as it is. But also, you need to tell your dad, this is a very big issue, and should have the right to know about this cheating. So get up the courage, and talk to them, or even write a letter.
Sincerely,
Mr. Swag