Brother is having an affair

No. You cannot make it better. Anything other than exerting subtle pressure to do the right thing is going to end badly.

I suspect he’s told you do you are compelled to do his dirty work. Don’t.

Stay out of it. Don't make yourself the bad boy which will probably happen

Your brother is a selfish creep. Your poor SIL and niece.

Not only is your brother a cunt for the affair but also for happily burdening you with the information.

His arrogance in telling you is vile.

You support your brother. He's been open with you for a reason. Obviously it's not a good situation but it's hardly unique. He's confided in you, take that on board and support him during what is going to be his marriage break up. He hasn't done this for no reason.

Going against the majority here but l wouldn't say anything. Obvs l'd let brother know l'm not supportive and don't want to hear any more about it but l know someone who did tell someone her partner was having an affair and to this day she regrets it. Wait for it to all come out , which from the sound of it won't be long, then be there for SIL if she wants the support.

Very hard and your brother is a prick for not only doing this but telling you

one of my best friends from school never forgave her father for the affair that pretty much changed her childhood. He’s long dead now, but the repercussions continue. Agree with you that he needs to leave, but sounds like he has no intention and then it will all blow up. That will have a far greater impact on his DD.

My Dad had an affair for years and gaslit my Mum over it. When they divorced and I found out it damaged our relationship so much it never recovered.

Maybe he doesn't care about sil but is he really prepared for his daughter to hate him forever over this "bit of fun"?

I think you should send him a version of your penultimate paragraph.

I know you've said that you feel you should persuade him to stop, but I doubt you can, and in your shoes I'd want to know no more about it.

Personally I'd send him a version of your penultimate paragraph but also say you love him dearly and want him to be happy but that you don't want to be complicit in his lying yo his wife and daughter and that you aren't going to discuss this with him again, and that if he does discuss it with you again you will tell his wife. That as much as you love him and will continue to love him and want a relationship, you dont want any involvement in his affair.

My brother was having an affair and treating his long time g/f (over 25 years) really badly, making her question her sanity. My brother is a bully, a bore and hates being told what to do but I can remember asking him that it was perhaps time he treated his g/f with respect and end it rather than play games, that 25 years together should mean respect. He rushed the end of the call, pretty much putting the phone down on me but he must've gone away and thought about it and ended it very soon after.
That's as far as involvement can go I think, encourage him to do the right thing.. Your brother won't thank you for getting involved and your SIL may shoot the messenger but like others say, certainly not lie for him in any shape or form.

Your brother is a lying selfish worm. Sorry you had to find out this way.

I think you should try and put your niece first. Your brother doesn’t care about you, if he did he wouldn’t have put you in this position.

Plenty of men do cheat on their wives. Any intervention from you in terms of telling SIL would probably be unhelpful.

I’d probably focus on trying to get to know your niece a bit better.

Whilst I agree, I have over my years done this twice with close local friends.

both stayed with him initially and I was made to be wrong.

Would I tell a friend again? Hmmm, I only have a Few close friends now in my 40s and we have all said yes, we would want to know.

a brother who lives far away, you have told him what you think. my fear would be he makes you to be the liar, as they do, and you lose your sister in law, niece and your brother.

I would not be part of deceiving my sister in law and so would let him know that he had to tell her or I would.

I would think so much less of him as a person.

Keep out of it other than telling him your thoughts about it each time it comes up in conversation. It’s not your secret to tell and if it coming out does negatively impact your DN then you will be the cause of it.

What is OW - 32, 33? If she wants DC she may have a very different view to his on where this should be going. Maybe point that out to him?

Simple. You say to brother that he has a week or you tell sil.

My answer would be the same. I might not tell the spouse but I certainly wouldn’t outright lie.

Tell your brother you're not going to lie if SiL asks you about it. He's been very, very unfair to you making you party to knowledge which could also affect your relationship with SiL and niece.

In fact he's been very unfair to everyone concerned. He needs to do the right thing, and quickly too.

I don’t know if I’d tell SIL, but I’d make it clear I wanted nothing to do with it, and I absolutely would not outright lie to SIL. So if she asked (he’s said she’s suspicious), or if he ever used me as an alibi and she asked about that, I’d say the truth.

This is exactly what I was going to say
How bloody selfish to tell you his 'secret'.

He’s fucking disgusting.

Your niece is your family too and she has this narcissistic twat of a father and now also an aunt that’s covering for him. He was bold enough to bring his affair partner around you and he feels no remorse in what he’s doing.

I’m sorry but I’d sure as fuck tell his wife, if nothing else so that you’re not complicit in this disgusting lie. Then she can do whatever she wants with that information.

I would. I wouldn't let my niece find out her father and her aunt have betrayed her. Nobody seems to have any morals any more.

What an arsehole. I'd tell him that he can tell her or you will because now he's making you complicit in his lies and I absolutely wouldn't stand for that.

Where was his loyalty and care to you when he opened his mouth and put you in this position? Selfish. He wouldn't have my loyalty, brother or not.

if this was a sister telling a sister would you really tell the brother in law? Doubt it. Why is it different when it’s a brother and a sister relationship?