It is obvious that you love both your mom and step-dad and because these are two people that you love and also your parents try to stay out of it.
I would talk to my mom and let her know that you hate that she's cheating. Explain that you are hurt by all of this and you don't want to choose who to be loyal to. Let her know how much you love your family and that you don't want anything to happen to your family unit.
Hopefully she will stop cheating and choose her family. She has put you in a terrible position. But again, because these are your parents I feel like you should try to stay out of it and let them work this one out on their own.
I hope this helps, but always do what feels right in your heart.