AIBU to let my nephew stay with us in holidays?

This^^

So at what point does the mum get to live her life?
He's 20, not 2

He is entitled to do that, yes. He's trying to do exactly that, his mum is actively trying to force him to come home and live with this twat.

OP, let him stay with you if you're happy to do it. . You're doing nothing wrong, you sister will have to get over herself and live with the choices she's made.

How is she putting a man before her kids? Her kids are adults and she is entitled to have a relationship and to live with her long-term partner. Her son still has his home, he made an adult decision not to live in it. This doesn't mean his mum has to remain celibate for the eternity.
My mum has a partner and I don't stay with them when I visit. I have a wonderful relationship with both of them but I have been staying with my aunt for years because I am more comfortable with it. I would never in million years expect my mum to not move on with her adult life for the sake of a few weeks a year when I visit. I am her kid, yes, but I am not a bloody child!
So the only thing the sister did wrong here is precisely to treat her son as if he was still a child.

Let him stay with you.
His mum has chosen her new bf over her own son, how sad.

Yeah move any man in, childrem are dispensable, he doesn't want to live at home why is that not enough

Another parent putting their needs first, and the new partner sounds awful

The partner is a bully and it is the start of coercive control. So keep lines of communication open and maybe send her a few things regarding this

Keep lines open with nephew and thank goodness he chats to your kids.

My sympathy for your sister's position has dwindled since reading your update about the partner. He doesn't sound nice at all and has evidently not made an effort to intigrate well into their family unit. I would tell your sister that you won't leave your nephew to spend holidays alone at uni. Tell her you will do everything you can to help them rebuild their relationship and she can come over whenever they both want to meet. She has every right to move on with her life now he has gone to uni but she can't expect her son to put up with shitty treatment from her partner. I had a stepdad and stepmum as a child and they are both wonderful and have added value to all our lives so it can be done if new partners make the effort to be decent.

I think this is a very delicate age tbh. I remember leaving to go to uni but still enjoying the comforting familiarity of being home again. It must be very difficult to have that removed because it doesn’t feel like home any more. Yes he’s “an adult” but really 20 is still very young in a lot of ways.

I think you’re doing the right thing OP by letting him stay somewhere that feels comforting and familiar. It’s a shame his mum doesn’t seem able to balance his feelings with her and bf’s. She should be grateful to you that you’re helping him through this tricky patch.

Given the update on the partner you absolutely should let your nephew live with you on the holidays.
The boyfriend should not be telling your nephew what to do. That’s not his place.

@CollectingSeashells Having read your update, I think it is entirely understandable that your nephew doesn't want to continue going home to a place he feels he isn't welcome any more. The new partner may ( or may not) have a point, possibly, about the time the nephew gets up or one or two other things you mention - can't really tell without further context - but it isn't the partner's place to try to 'parent' your nephew. But given the other things he says and does it sounds very clear that the partner does not want your nephew in the house. It is also sad that your Sister can't see it for herself.

I think you should support your nephew and tell your sister that there is a better chance of her repairing her relationship with her son, if you let him stay with you rather than tell him no and he will stay at his Uni address instead.

I would, however, make sure that you discuss the ground rules with your nephew before you agree to his proposal - things like what time you expect the house to be quiet if he has friends in, wants to put a washing on etc. In other words what you are happy to live with? It IS possible that he does make a lot of noise with some of the activities the partner is getting onto him about?

Poor Lad! I feel for him

He still has a space to come home to.
Mum has the right to move a long term partner in... her son is at Uni FFS and mostly not there.

If your sister wants her son to stay at her house in the school holidays, she'll have to listen to him about what is upsetting him and make some compromises, which would reasonably include asking her new partner not to micromanage his partner's adult child's decisions. Otherwise the son is going to stay elsewhere, as he has already demonstrated by staying at school. If everyone in your household is genuinely happy to have your nephew there then that sounds like a good temporary solution; sis and her partner can have things all their way and son can be safe and happy.

As far as your "interfering", this isn't logical: you invited him for his cousin's birthday, he then asked you to consider his staying on a longer-term basis. You didn't approach him to move in. His staying with you could be a very helpful temporary pressure valve to both him and your sister. At 20 he could disappear and she'd never see him even if the current partner works out; this way she can still spend some time with him without having to row about the new unilateral "house rules".

It’s none if his business. Hell would freeze over before a newly moved in partner started telling my son what to do

I think everyone needs to accept that everyone in this situation is an adult. Your nephew needs to accept that his mum is an adult and is entitled to live with her partner, especially as he himself is an adult who is away at university a lot of the time. Your sister needs to accept that your nephew is an adult who can make choices about where he lives - and that you are also an adult who is equally entitled to make choices about who lives with you in your home. And that’s the long and short of it, really…..

Absolutely in a heartbeat I’d offer my nephew a place to stay in those circumstances and in fact a similar situation did happen in my own family involving a cousin of mine. Make sure he knows he is always welcome.

This exactly. The BF sounds like a twat. And very convenient for him to be driving her son away not long after moving in.

What do you think of your sister’s BF?

Yeah, don't blame nephew for not wanting to live with his mother's partner... and at his age, he doesn't have to suck it up, he can vote with his feet. His mother is obviously viewing this as a slight on her partner and is sore about it, but she needs to get over herself thinking she can dictate what other people do.

Actually he seems to have the measure of the partner. He sounds horrible.

Try to reframe it as if he stays with you then he can get to know partner. Christmas will be awful for him in student digs on his own.

He sounds awful. He needs to tell his mum all of this.

Plenty of reason to strongly dislike him.

And worrying behaviour that could turn on your sister at some point.

I wouldn't be astonished if he is trying to push nephew away as a form of control. At the very least it's very disrespectful to his partner's child.

Whatever her reasons, she doesn't get to dictate where her adult son lives. If he stays with his auntie he's still with family and his mum knows where he is - he could stay in his uni town and move in with other people she doesn't know (which of course he's free to do because he is an adult with agency).

She is the one driving a wedge with her demands and accusations, and if she doesn't see sense and quit it she could end up with a son going low contact with her and a damaged relationship with her sister (OP) too. And to what end? Nobody is going to win from her attitude!