Originally Posted by ModelPerfect

I called that one before I finished reading the first paragraph. No shit your gf and her family are mad at you. Unless the dude was your best friend, you had no right, and being that he "was planning on breaking it off anyways", there was no reason to regardless. Not to mention you already said in the title that you only "somewhat knew".

I despise cheaters, but look at it from another angle. Let's say your best friend was cheating on his girlfriend. Would you tell her? You have stronger ties with your friend...not some random girl...if you told her, you would lose a friend, guaranteed, because you betrayed his trust. The fact that he betrayed his girlfriend's trust doesn't equate to you, because she isn't anything to you compared to your friend. So, how would this situation be different? By default, you have to have more loyalty to the sister rather to some random dude. You don't have to condone it, but to outright spill the beans is to betray your girlfriend's trust. And for what? To stroke some drama queen ego based on unsubstantiated facts to some random guy who's going to break it off anyway? Doesn't make sense to me.

It would have been better to talk to the sister about it...