screw all these adults who say "stay out of your parents lives" because most likely they cheated or were caught cheating before. First, talk to your mom, unless its a stepmom and your real dad then fucking scream at the top of your lungs what happened. But if shes your biological mom. catch proof first. then talk to her but don't show her the proof. then if she denies it tell your dad. I say get proof first because if she finds out you know she might be more careful about it.
If it were me, I'd tell her I know about it. I'd ask her what lesson she imagines she's teaching her child(ren). I'd tell her that her marriage may be none of my business but I'd like to know how she could do that to my dad? I'd tell her that I love her but wrong is wrong! It's time to s#!t or get off the pot.
1. its none of your business 2. u could speak with your mom 3. Don't tell your Dad because 4. its none of your business 5. don't take sides because 6. its none of your business 7. I know you love them both
I agree with cookiroma. My mother did the same and I was the only one that knew for years. Talk to her about it, tell her how you feel about it. Sometimes as parents we make mistakes and don't realize how they can affect our children, and as a child looking up to your parents, it's normal to put them in higher standings. Remember, your mom is only human. Good luck and I hope everything works out for the best!