You have discovered that your parents are human. Stay absolutely out of it, neither be part of Mom’s cover up nor run the news to Dad - Explain that to Mom. You are not responsible for your parents’ relationship. If or when it all blows up you do not want to have been involved because that will affect your relationship with both your parents. My own mom had a affair that came to a blowup when I was in college. I was not aware of it until then. Mercifully to me, my concerns at the time were on college and not being home all the time was best for me. Eventually the affair didn’t work out for Mom (the other guy had too much to lose to get a divorce he said). My parents got remarried “for the younger kids” (I am the oldest of 4). A couple of years later Mom did it again and this time Dad got custody and that was the end of their relationship (no more remarriages). My Mom, like yours I assume, has some occasionally serious flaws but she’s still my Mom. I always was closer to Dad but I got parts of my personality from both. You don’t get to choose your parents. You have to make do with what you have. Let your parents sort out their own relationship.

Sounds like you have some pretty juicy blackmail material. Use it wisely.