Brother is having an affair

I don’t think my brother would tell me because he knows how I would react. Knowing something like this would consume me. I think he’s put you in an awful position and when she finally finds out that you knew too it will feel like a double betrayal. Give your brother an ultimatum to end the lying either end it with the lover or the wife.

If my brother told me this directly and in person and was so blasè about it, I would tell his wife.

If someone knew similar about my partner, I would want them to tell me. I wouldn't expect it and I'd understand why they didn't but I'd want them to.

My exh had an affair. I knew some of his colleagues independently of him but we weren't friends - we did a hobby together. They knew abut his affair long before I did. I wasn't cross with with for not telling me but the burden they felt was clear on their faces when I told them I knew. I wish they'd felt able to tell me.

It may not be for you to tell SIL, but as a woman and a human being it is up to you to persuade him to stop the duplicity.

A winning family.

What luck for your SIL and your niece.

Not worth discussing.

OMG don’t be the one who tells the sister-in-law!!! The Muppets who are giving you this advice are exactly that - muppets
That’s his job. Stay well out of it. Otherwise literally everyone will then blame you. DN, SIL, and your brother.

Just why has he burdened you with this and not other family exactly?! What an awful position to put you in. I agree it isn’t for you to tell SIL

The trouble is you are both now in the wrong. Lying by omission is still lying.
You are effectively condoning it, siding with your brother’s secrecy.

It’s the worse feeling being cheating on and finding out no one was interested enough in your feelings to tell you the truth.

I get you feel it’s not your place though and that your brother is not a one dimensional human. Especially fucking up the marriage this close to Christmas. Work on the brother and the girlfriend if you can to come clean. Fresh start in the New Year or else.

I’d be very clear on just how disgusted I was with him. I’d also tell him he had no right to burden you with this. I would tell him that while you won’t be volunteering any information, you absolutely will not lie to his wife if she asks. I’d encourage him to grow up and stop being such a selfish bastard.

If any of my brothers told me something in confidence I would not be breaking that, whether I agreed with whatever they told me or not. Who knows what goes on behind closed doors in their marriage.

Op I think you have received some extremely harsh comments on here. At no point do you say you are ok with it or agree with what he’s doing. It’s a shit situation for him to put you in and there is no perfect answer.. so you need to choose the one that has the least shit outcome in the short and long term for you. As you say, he’s your only living relative and you’ll want to retain a relationship with him long after their marriage blows up. So I agree with not saying anything. You can guarantee that you aren’t the only one that knows. There might well be good friends of hers (and his) who know who aren’t saying anything either.

As for those saying he deserves to have his access to his DD restricted - why? So effectively punishing the DD for her father’s actions? Apart from anything else, whilst he may not be a model human being - ‘restricting access’ sounds like something out of the 70s and 80s divorce courts and isn’t legal anyway. And yes my Ex did have an affair but I would never have tried to stop him seeing his own DCs however much he hurt me.

Op I hope it all comes out soon. It sounds like he’s the master of his own misery anyway. You don’t need to get involved.

Must be really hard for him. He's clearly smitten with this other woman. This'll end as she'll probably get bored of him. If he doesn't want to ruin his marriage, he needs to be more careful.

Begs the question what is the point of the thread. If you are not bothered what your Sister in Law thinks of you and you consider petsistent cheating on her by your brother as a mistake and his past supports trumps his deceitful and dishonest behaviour because people are 'complicated' then what's the actual problem for you. You obviously have it sorted in your own mind, so crack on. It really isn't complicated. He's a cheat, you know he's a cheat and that's it.

Not being the only one will make it worse for her, BTW. Hopefully your only living relative bar you children gets what he deserves. A bedsit on his own with limited access to his daughter.

People always say they want to know but in real life the messenger gets shot particularly when so many couples decide to have another try.

She can and she absolutely should as it will be far better for everyone involved. The trouble is, it isn't the easy path. The easy path is leaving it to go on as is and not to worry about the impact that will have on the niece and sister in law. I feel very sorry for both of them, they will feel so let down by the father and now the aunt. I am very certain that if anyone was in their shoes, they would want to know. He's already done the devastating damage to his family, the only way to lessen that is to make sure they no before it gets worse. I don't know why anyone would be bothered about preserving a relationship with such a pathetic person.

The OP can’t tell the SIL, realistically she could then lose her relationship with both of them.

Just keep pressuring your brother OP but it sounds like the wife suspects anyway so it will all come out soon enough.

If you ask 100 people who have been cheated on what was the worst part of it they will nearly all say that they felt complete fools that other people knew and they didn't. The humiliation of others knowing when they didn't seems to be worse than their partner cheating. Most know that their partner is a cheat, or suspect they are, or that there are issues in their relationship and the cheating doesn't come as the biggest surprise. But the lingering sense of betrayal of friends and family not telling them when they knew is nearly always said as the worst thing.
There is no way your sister in law will forgive you or understand your position, and why should she. She will get custody of your neice and it's highly likely she will restrict access. And why wouldn't she. So your loyalty to your shit of a brother is going to have consequences. He's much more likely to forgive you if you tell, than she is if you Dont and it comes out you knew.
This is all on your brother. He shouldn't cheat and shouldn't have told you he is cheating. But he has, and you have a choice to make that will have consequences.

Not your business.

I think that people having affairs like this get themselves into a kind of frenzy. They are so caught up in the infatuation, the excitement and the indulgence in whatever thing that feels missing in their life that the affair is representing to them, that they often don't want to really think about the very real consequences of their behavior. They get so practiced at compartmentalization that they tend to conveniently forget the very real impact of their actions. You've done the right thing by pointing this out, but he probably doesn't want to hear it. For him, the easiest thing is to carry on compartmentalizing and, "having it both ways." It's almost like an addiction where he knows what he's doing isn't good, but he wants more and he feels powerless to control it, even though he knows objectively that it won't last. He's chasing a high. He's basically enjoying what feels missing from his life, whether it's youth, freedom, excitement and adventure or whatever it is that this affair is representing - without experiencing the terrible loss and heartache that being found out or making the courageous decision to end the marriage would cause. In short, he's being unrealistic, short-sighted and selfish.

I think that all you can do is to keep pointing out that he's doing even more damage to his wife and daughter by handling it this way and not taking the bull by the horns and ending his marriage. I don't think anyone can say you shouldn't be loyal to your bother, but you definitely don't want to be in the role of enabling his behavior.

You could ask him what values does he want to guide his handling of this and how does he think his behavior is lining up with his values?

but there's never "no reason". He doesn't sound like a serial cheater who shags a different woman every Friday and comes home to have his laundry done.

It's 2 people who are unhappy in the marriage. They are both equally responsible for the marriage being over.

I stand by what I said, he's not protecting his daughter and that's a poor excuse, he should just ask for a divorce. That will blow up in everybody's face, and it will make it look like he's the guilty party.

The real sadness is that your brother told you, OP, because you can't "un-know" it.
However, this is none of your business, so you need to keep out of it and just be available to listen to either/both of them, without taking sides.

Oddly he wouldn’t cut me off for speaking my mind

How very big of him.