Trouble in Paradise

If I had a brother I would let him know what they said and let him decide from there how he wants to handle it. But you also know best how your brother would feel or what he would want.

I don’t have a brother or sister but I have learn the hard way not to intercede in relationships. People always make up and you end up looking bad. At the end of the day you don’t know what happened a month ago that made her feel that way. If you really want to talk to him I feel like maybe saying you overhead a conversation that him and his fiancé were having issues a month ago and see what he says. I think if you straight up tell him you’re opening the door for a lot of drama to come your way.
Good luck.

If it were my brother I would just say I over heard his fiance say they had a problem a month ago and you wanted to make sure he was feeling okay. See what he says.
If he talks about it then good, you get his side of the whole story.
If he doesn't I'd personally say something like well it seemed like (fiancee) was still upset over it and possibly having cold feet. I wouldn't just come out and say she's leaving you.
People tend to be dramatic when emotional there's a chance she was being dramatic for sympathy. Make it clear you didn't hear the whole conversation and just wanted to make sure everything is okay.
Be prepared to be the awful a hole that ruined an engagement though.

I agree with Kelly

If I was in your situation, I would definitely tell my sister what I heard (I have a sister) & I would certainly expect her to do the same for me. How can you not tell your brother? Of course, you should be tactful about it & approach it gently, as pp mentioned. But I would never keep that for myself. In fact, I would probably ask FSIL when I heard it, what’s going on.

Stay out of it. For all you know, he had done something that really upset her, legitimately, and they worked it out. This happens to a lot of people during their engagement period. But it is none of your business. She did not move out. Future SIL was talking to one person she trusted. You should have made your presence known immediately when you got where you could overhear what they said. So now, act like you never heard it. It is not your business. And most especially, it is not your business to bring tales to your brother about his future wife. . . . There is nothing actively bad that she is doing. All you know is that something happened that needed to be worked out. And his part or her part in their private relationship or marital issues, is none of your business. If you do bring it to your brother, first thing he will do is get angry at you.

I’ve had points where I wanted to leave my fiancé too because he treated me bad and yes I would vent to my bff about it (granted not that close to the wedding). It doesn’t mean I don’t love him, relationships are just hard and people have to work on things over time. I wouldn’t interfere.

I will start by saying I am not close with my brother or my SIL. My SIL and brother had always had a very rough relationship. Both had cheated on each other, my brother has a temper, SIL is just a really unpleasant person. They had a baby at 21 and regularly used him as a threat either toward my parents or each other if the one threatened to leave. Just a millions ways of nasty. While no one in our family cared for the relationship, everyone was cordial to her and we never spoke ill of her with my brother around. They have now been married for 4 years and have 2 more boys together. It took a lot of work, a lot of therapy, both together and separate, and she is still not my favorite person, but sometimes people grow up and things get better. I think if she were really that close to leaving, your bother is probably already aware. We personally stayed out of it whenever their relationship was involved because it's really not our decision to make and it could destroy your relationship with your bother. No matter what you decide, tread very carefully.

I think it very much depends on your relationship with your brother. If you have that type of relationship where you can tell him your concerns without him seeing it as you are talking down to him or his relationship, I would tell him. But if there is any sliver of a chance that he could get upset at you, I would stay out of it. Sometimes it is best to stay out of it and let the relationship runs its course, wherever and however, that may be.

I agree with pp's. Provide your brother with the information, but then let him make his own decision.