[My girlfriend and I are in] a serious relationship… We've been together for 4 years and just a month ago I suspected that she’s having [an affair], [specifically, that she was] cheating [on me] with my brother-in-law, but I said nothing until my sister [began] suspecting [that] her husband [was cheating] and we both felt the same thing after we talked. What should I do?
I wish you had explained why your sister and you believe that her husband and your girlfriend are having an affair with each other. Cheating is a very big deal, but when you accuse someone of having an affair with a family member, you introduce a new element of offense into an already despicable situation. And while I typically recommend communicating openly about things, I’m concerned that it’s going to be virtually impossible to back-peddle from here if they are wrongly accused. For the purpose of providing you with some sort of answer, I’m going to presume that you have damned good reason to think that your girlfriend is schtooping your brother-in-law.
I do believe that relationships can move past an affair, particularly because I believe that an affair can be a symptom of a deeper problem. Make no mistakes, it’s a dysfunctional and selfish coping mechanism in such cases, but I’ve seen people genuinely move past the pain and betrayal and come out the other side.
That said, your girlfriend’s choice of sleeping with a) A man, and b) A member of your family, makes this beyond complicated and in my opinion, beyond forgivable. You, my friend, need to kick this one to the curb and never look back. And if your sister makes the mistake of staying with her good-for-nothing husband, I’d resolve to never be in the same room with him again.