I get far too many letters like this one. The situation is awful for the kids involved. You shouldn’t be playing detective. It’s not your place to get involved with your parents’ relationship. But I do understand that you love your dad and don’t want to see him hurt. He already has suspicions, but he isn’t pursuing them. That leaves you wondering if he wants to know.
You have at least two choices: You could decide to ignore the situation and leave it to your folks to work out their problems. Your mother isn’t being subtle. Your dad will eventually catch on if he wants to.
Or — you could tell your mother what you know and let her know that she can tell your dad herself or you (and perhaps your sister) will tell him. I’m sure this won’t go over well but if is an option if you feel you can’t stand keeping silent. She has already shown you how she will respond. Be prepared for more of the same.
I can’t tell you what is best. You know the situation. You know your parents.
I wish you well,