Should I tell her that she's being cheated on?

My fiances brother told him the other day that he hasn't been happy with his girlfriend (of almost 10 years) for a few years now. He stated that he's in a relationship with someone else. My fiance said that he should break things off if he's not happy but said it would crush her. I wasn't even supposed to know but my fiance told me anyway. With all his girlfriends mental issues he thinks breaking up would hurt her more then her finding out he's been cheating. She I tell her what I know? Is it nor my business? I would want to know but I don't think she would believe me anyway. it's a sticky situation. I also have a hunch that my mother inlaw knows her oldest son is cheating on his girlfriend but is okay with it because she never liked her anyway. I know it's not my business but I can't sit around the Christmas tree pretending everything is fine. She knows I don't like my fiances brother or he much either but it still sucks knowing what I know. 

T J2020-12-03T11:16:49Z

Think about this, and how you can suggest to her, that she is being cheated on. Do not be direct, just maybe point out some signs of a cheater, maybe she can put 2 and 2 together.

Emily Rose2020-12-03T05:23:07Z

Wouldn't you wanna know? i think you should tell her just because if it was you and your man made you look like a fool and everyone else knew then you would wanna be informed. The truth does hurt sometimes but its gonna hurt more if she finds out that you guys knew and didnt tell her.

?2020-12-03T04:37:04Z

She has a right to know, but you don't have the right to tell her directly. Give hints or tell him to be honest or you will!!! As I appreciate you being concerned but it can cause for issues between you and your fiancees brother, and family.