Yikes! I guess your Dad was better at hiding the affair than your husband. Is that really better?
This is a totally different topic, but it's in my head and maybe it relates. My son, 24, is driving back to New Orleans tonight, where he grew up, for the funeral of the 4th friend in less than 1 year. The boy, 26, father of 2 children, died of a drug overdose. My son ran with a pretty wild crowd, but they were all suburban kids who were raised in pretty well-to-do families. Everyone agrees that if my son had not been busted for drugs when he was 19, and spent three years in a recovery/rehab program, he would be among the dead. When discussing it with my younger son,18, tonight, S18 said it seemed like the ones who were wilder younger, and got caught were the lucky ones. The ones who never got caught did drugs until it killed them. Perceptive kid.
Which leads me to the question: Is it better to get caught? If you get caught and you have a huge blow-up, then you do something about whatever was making you so unhappy to begin with. But if you get away with it, then the marriage doesn't change, and you get to stay in an unhappy relationship for life.
So does that mean you should out your father at this late date? I think that the existential moment is long gone. It's no longer a crisis. At this point, maybe your Mom would be happy that he came back to her and never felt a need to say anything. Maybe he'll laugh about sowing some wild oats. Would your siblings really care now, if there is no threat to the relationship? Or would your Mom lose it, and hate him for life? Would it cause her everlasting anguish over something she could have lived happily without knowing? Doesn't seem valuable to bring it up at this late date.