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AIBU?

Share your dilemmas and get honest opinions from other Mumsnetters.

Brother is having an affair

116 replies

strayingsibling · 23/11/2025 19:45

Met up with my brother for a drink this afternoon, first time in ages. We get on well but live a few hours apart. He was at an event near me last night so we arranged to meet up.

He’s late 40s, I’m a couple of years older. Our parents are dead and we have no other siblings. He dropped the bombshell that he’s seeing a woman he knows from a sport they both do, it’s been going on for almost 6 months. He was with her at the event last night. I knew he was unhappy in his marriage and was actually expecting him to tell me he and SIL were separating, but he says he has no intention of leaving her as it would be too disruptive for my niece, who is 14.

He says SIL is suspicious but he has been lying to her and has denied it when asked. SIL is a nice woman and we get on well. I know they are not right for each other but I feel pretty strongly that she does not deserve being gaslighted like this. I also think that the real reason he’s not being honest is that it’s highly convenient for him to have SIL at home to care for DN while he swans off with his OW.

The OW is 15 years younger than him and he says he knows it won’t last but needs to enjoy life (neither of our parents made it past 60 and this has affected his outlook on life). He’s quite starry- eyed about her.

I feel that it’s highly likely SIL will catch him out.The impact on my niece, knowing her father has been lying to her mother, is going to be devastating when she is already dealing with adolescence. It will probably ruin their relationship forever but he can’t seem to see this. I told him he needs to end his marriage and the affair, accept he’s had his fun and it’s over now and deal with his marriage breakdown like an adult. Maybe that way he can limit damage by never confessing the affair.

He’s my brother and I love him and I want him to be happy. But, honestly, what a twat he’s being. AIBU to feel that there is nothing I can do to make this better? Does anyone have any advice?

OP posts:
ledmeup · 25/11/2025 02:56

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.

strayingsibling · 25/11/2025 17:56

BobblyBobbleHat · 25/11/2025 02:46

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 “pathetic person” is my only living blood relative apart from my own children. He may have made a big mistake here but he has time and again proven himself to be a rock during very, very difficult family circumstances. People are complicated. Perhaps our relationship would survive anyway if I did tell his wife, but I am not that bothered by what she thinks of me if she finds out I knew. When it all comes out I am sure I won’t be the only person who knew and didn’t tell her.

OP posts:
ClareBlue · 25/11/2025 18:22

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.

pocketpairs · 25/11/2025 18:41

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.

beasmithwentworth · 25/11/2025 18:42

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.

booboohoohoo · 25/11/2025 18:46

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.

YodasHairyButt · 25/11/2025 18:48

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.

SmalltownCEO · 25/11/2025 19:14

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.

Sheaw · 25/11/2025 19:18

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

curious79 · 25/11/2025 19:22

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.

FairWarning · 25/11/2025 21:05

A winning family.

What luck for your SIL and your niece.

Not worth discussing.

strayingsibling · 25/11/2025 23:43

Sheaw · 25/11/2025 19:18

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

Edited

We don’t have any other family.

OP posts:
strayingsibling · 25/11/2025 23:47

ClareBlue · 25/11/2025 18:22

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.

I am not bothered what she thinks of me. I am bothered about the impact on my brother’s relationship with his daughter and his prospects of an amicable a divorce as possible. It’s not about me.

OP posts:
Swiftie1878 · 26/11/2025 07:41

strayingsibling · 25/11/2025 23:43

We don’t have any other family.

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.

GreyCarpet · 26/11/2025 08:01

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.

ItsInTheSingingOfAStreetCornerChoir · 26/11/2025 08:12

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.

BunnyMcDougall · 26/11/2025 08:34

strayingsibling · 25/11/2025 23:47

I am not bothered what she thinks of me. I am bothered about the impact on my brother’s relationship with his daughter and his prospects of an amicable a divorce as possible. It’s not about me.

If you’re bothered by the impact on your niece, I would advise you to think very carefully about your actions (or lack thereof). If she finds out that you knew, and took part in keeping it a secret, it might not end well (speaking from experience—things came to light 10 years after my parents’ divorce).

BunnyMcDougall · 26/11/2025 08:37

ItsInTheSingingOfAStreetCornerChoir · 26/11/2025 08:12

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.

Agreed. It’s like the brother has been carrying a big weight on his shoulders for the past 6 months, and has just thrown it at his sister. “Catch—you can carry this for the coming months/years.”

Isthisreasonable · 26/11/2025 08:45

Keep pointing out to your db the worsening impact of not splitting from his wife on his dd.

She's rapidly heading towards GCSEs

She's getting more and more understanding of relationships as she matures and so less likely to believe whatever story he comes up with

She's probably starting to have boyfriends and will be learning what it's like to be dumped/cheated on

She will be developing her own opinions and they may not tally with his

If by some miracle he doesn't split with SIL until she's an adult, it will have a huge impact on her as it will re-write her "happy" childhood and impact on her own relationships in the future.

None of this is positive for your db and his future relationship with his dd. If he really cares about her, the sooner he splits from his wife the better for all concerned. All you can do is keep reminding him of it.

Lennonjingles · 26/11/2025 08:45

Op, what did he say when you told him to end relationship, like you I wouldn’t be happy, but I wouldn’t want to be the person who tells his wife, he’s now put you in an awful situation, let’s hope he does the right thing, whatever that may be.

ClareBlue · 26/11/2025 20:30

strayingsibling · 25/11/2025 23:47

I am not bothered what she thinks of me. I am bothered about the impact on my brother’s relationship with his daughter and his prospects of an amicable a divorce as possible. It’s not about me.

OK, the high probability is that relationship that your brother has with his daughter will never be the same again, it will have an extremely negative impact your neice and will impact on her ability to trust men as an adult and she could end up repeating the pattern herself by chosing men who cheat, she will look at her dad as someone who cheated on her mum for the rest of her life and question every excuse he made to spend time having the affair and mot being with tge family, she is highly likely to internalise this with feeling her Dad priorities were not her or her mother. She will most likely see your brother as someone who didn't have the moral courage to end his marriage before finding another sexual partner. She will most probably look at her aunt as complicit in the deceit and hurt and will struggle to understand how those close to her could act in this way.
That's the most probable outcome when adult's who have children are 'complex' and the complexity of their character manifests itself in cheating and deception.
It's not about you, you're correct. What do you actually think is going to happen, what does your brother actually think is going to be the outcome of his choices.

ClareBlue · 26/11/2025 20:35

And amicable divorces rarely happen when cheating is involved. It's all part of the delusional self centred actions of people who cheat. They think everything can be managed and rationalised when the inevitable separation happens. It very very rarely happens.

reversingdumptruckwithnotyreson · 27/11/2025 17:36

strayingsibling · 25/11/2025 23:47

I am not bothered what she thinks of me. I am bothered about the impact on my brother’s relationship with his daughter and his prospects of an amicable a divorce as possible. It’s not about me.

His daughter will probably resent him and lose respect for him even if they still maintain contact.

It is what it is, he chose his penis over his duties as a parent.

Kidsgotothatschool · 27/11/2025 17:45

Your brother is being abusive, he is putting his wife’s mental and emotional health at risk through gaslighting, minimising and lying. If he and his wife are still having sex he is also removing her right to informed sexual consent and putting her at risk of STIs. He is most definitely removing her personal agency.

I actually get why you feel you can’t tell her but i wouldn’t be able to stand by and watch the mother of my niece, wife of my brother being abused.

I’d be laying this all out to him and telling him you can’t support this and he needs to end his marriage.

I have seen the damage cheating can do to a betrayed spouse and the long lasting impact on the children, your brother is behaving appallingly. I couldn’t stand by of it were mine.

SweetnsourNZ · 12/12/2025 04:10

wildone345 · 23/11/2025 20:07

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?

Probably because you would not be as close to the bil. It does change the dynamic. He's a real piece of work as if sil finds out she knew the friendship will be over, if she tells sil she will be the messenger and we all know what happens there. Chances are this isn't his first affair either. And bet the silly OW doesn't know she's just a temporary plaything.

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