To let my niece stay when she refuses to see her mum?

I think it's obvious your niece felt pushed out at 17 - your sister should have made the effort to do things with her without her partner present and should have understood that was what your niece needed and wanted.
But that was in the past and I also think your niece is being too extreme by cutting her off altogther. Also, the decision your sister made to have another baby should not cause this much offence - your niece is growing up and setting out on her own life and your sister is entitled to have a baby if she chooses.
Her staying in her uni town isn't upsetting as lots of students do that, my son included - he preferred to stay with friends as much as possible. But coming back to stay with you instead of your sister is a snub, so it's understandable she feels hurt.
The only way forward is really to talk and for your niece to start making amends with her mum, even if that means meeting her every now and then and not staying with her. I think letting her stay is ok, but only if you are working towards getting her to do that and making her have conversations about it.

Well that's exactly my point. You have no idea how the woman actually treats her daughter

Team niece here. Your sister decided to dance all over your nieces boundaries at a difficult age mid a levels. Your sister is completely unreasonable to expect you to not let your niece stay. Blending families needs to be done carefully over time, your sister has been an idiot.

This. I’m Team Niece too. Your sisters actions, while not deliberate, were thoughtless and selfish, and I can totally see why your niece feels the way she does. Be there for her and help keep the comms channels open. As your niece gets older she may be more accommodating about your sisters view of things. She may not of course. Your sister needs to take a serious look at herself and her actions and ask herself why her daughter feels like she does.

Your niece is a brat

Of course, what else?

Breaking off all contact forever merely because her mother dares to continue a life of her own is absurdly self-centred and destructive.

I’d do the same as you. I don’t think she sounds spoilt. I think your mother having a baby when you’re 19 would be hideous and bring home from uni with a baby around would be horrible.
Is her Dad around ?

Agree with this. And it does sound a bit like everyone- including you, OP- got used to your sister being single and don't like the fact that she now spends time with her partner.
I think the way your niece is wording things does indicate she's a bit spoilt, and you don't sound at all neutral tbh.

I think the only way to manage this is to say to your sister that, by letting your niece stay with you, you hope to have conversations with her that will lead to a softening of her attitude to her mum. Let sis see it as a positive thing that might lead to a reconnection, whereas nothing will change if niece remains in her uni town.

I feel for your niece. She has been pushed out. I think this is about so much more than where she spends a couple of weeks.

You are keeping her in contact with the family/cousins. Those bonds are important. Your sister should recognize what you are doing.

You're basing that purely on what the OP has said in this thread?

My niece is such a nice person but she is adamant she wants nothing to do with her mum, her partner or the new baby.

So, not "such a nice person" then.

I'm full on your niece's side. I wouldn't entertain your sister trying to dictate who stays in your house

I don’t think she’s spoiled. I admire her for having the confidence to say what a lot of children in blended families don’t have the voice to say. Support her, your sister just needs to learn actions have consequences and while it was her decision to have another child, it’s her daughter’s decision as to how she acts on how that makes her feel.

I agree with this i think.

My ex left me several years ago and I've never introduced any other man to my kids. I'm passed the age where I'd have another/or woukd want to, but if I met someone and did want tgis I know my kids would be horrified, even tho their dad was with someone else straight away.

Your sister introduced no man for about ten years? That's a long time to put your life on hold.

I can see both sides. But how heartbreaking for your sister to be ditched by her own DD after she finally met someone serious after years and yyears.i hope they reconcile one day.

YANBU for having your niece to stay.

It’d be U to seek to share your views with or influence DN over what she thinks, feels and does about her mum / your sister.

This.

If you grow up with parents that make bad choices, extended family become a lifeline.

It sounds like your niece has the measure of your sister, I think it’s lovely she can stay with you. You absolutely should prioritise your niece, it sounds like she’s got no one else supporting her and backing her. Your sister’s created this mess, she doesn’t get to call the shots anymore.

I would bet my bottom dollar that there is more to this, from your niece’s point of view. It’s not about her being spoilt 🙄 It’s about her feeling sidelined and long before the baby came along. Your sister moved in a man that your niece never wanted to live with. This shouldn’t have happened, and your sister could have waited until her daughter had left home.

YANBU and you sound like a brilliant aunt. Well-done for supporting the vulnerable person in all of this.

In your shoes, I’d absolutely have the niece to stay. I’d play the neutral role for now. Your niece has offloaded how she feels, but she may need reminding occasionally that you’ve your own relationship with your sister, so don’t get drawn into sides. Your sister may see it differently for now, but you’re providing a ballast for your niece, and you want to have a loving relationship with your baby nephew.

Im not a big believer in going NC or cutting family off for life, so if it were my niece, I’d hope she would know there was someone looking out for her. I’d be hoping that in time there would be a thawing, or some sort of new relationship will grow.

Would you consider suggesting therapy to the niece? I don’t think she’s spoiled; I think she’s reacted to the shock change in her small family unit, and she needs time to mature and figure out the whole new dynamic. Give her time, be there for her, but draw a line on her criticising your sister while she is a guest in your home.

Are your parents still alive? Does she have a relationship with them or the wider family like other cousins?

YANBU OP, and neither is the niece. Your sister has basically replaced her and as a pp said, likely just wants a free babysitter, this all sounds very controlling and possibly the tip of the iceberg if your DSis can even think it’s ok to tell you not to let niece stay. Niece shouldn’t feel pushed into talking it through with her mum, that will push her away, she’s holding boundaries and that’s ok. If your DSis posted on here “should I have a baby at my age I have a 19 year old and moved my new bloke in over her head without considering DDs feelings, refused to fund my existing child’s uni, and yet magically had money to bring another child into the world?” she would rightly get pasted. But the “we hate adult DDs” are out in force right now, I think a lot of them are triggered by the DIL thread they’re currently piling onto elsewhere. It’s not ok to use housing to try and force a reconciliation.

I’m conflicted a bit on this one.

On the one hand, your sister has every right to live her life. She sounds as if she has been very disciplined about giving your niece a good life and doing all right things (up until new partner). If your sister wanted to have another child she would not have been able to wait until your niece was an adult to do that. I don’t think she should have to forgo having another child to solely focus on a child that has left school, that’s an unrealistic level of sacrifice.

It sounds like she has gone about in a really poor fashion and that has really hurt your niece. Instead of owning it, she’s trying to control the situation by dictating that you can’t see your niece. It sounds as if she needs to hear why your niece is hurting without directing the blame at her - hard I know! Would your niece see her if it was just the two of them?

Without all the info, I do think your niece is being somewhat extreme. Understandable as she is still young. One thing that stood out for me is her sitting her mum down and telling her her feelings about her having IVF. While she is absolutely entitled to her feelings, what did she expect to happen - that your sister would not do it on her request? That would be wholly unreasonable.

I feel your sister has a lot of work to do to understand that she can’t control this, and she needs to accept her actions have deeply hurt your niece.

For your niece, it think it is important to help her see where her expectations aren’t fair or realistic, and where they are, so she can set healthy boundaries. I know it’s popular on Mumsndt, but I think cutting off your sole parent isn’t healthy unless in significant circumstances. Your sister isn’t abusive and if she is able to own her part of the process then I think it would be helpful for your niece to understand that holding boundaries (such as seeing her mum independently of the partner/child) is important, but that it could also be useful to consider whether all of her expectations are realistic. If your sister is generally a good person and has been a good mum, I think it would do more harm than good to completely cut her off at 20 and never look back.

I have mixed feelings about this one. I think your niece’s anger towards her mother for prioritizing her mother’s partner is valid, but I bristled at, “You’re too old to have a baby and the age gap is ridiculous.” Guess what, kiddo – your mother’s decision to have another baby isn’t subject to committee review, and you look childish as hell for refusing to see your own half-sibling even one time. It comes across like “Wah, wah, I’m a grown woman who’s jealous of a literal infant.”

Should you (OP) let her stay with you? Yeah, sure, go for it. You and your niece are adults and don’t need your sister governing the terms of your relationship.

Obviously circumstances are different for your sister and niece but the change in personality, the idea that your niece didn’t need as much one to one time as she was starting to have her own life all seems convenient for your sister’s relationship. It’s also not true, this was probably one of the biggest and most important transition times in your nieces life - leaving school and going to uni - a mum or mother like figure would definitely still be needed!

Yes, it did improve with time. I moved out for uni instead of staying home as planned to get away from the stress of it. The man she moved in was awful, and it was like her personality changed as a result. In time she saw him for what he was and after that is when she came to her senses really. The sisterly relationship survived - my aunt is very no nonsense, plain speaking and whilst it was strained at the time they still spoke and it’s strong now. We think it was a kind of mid-life crisis as she’d been through losing my dad years before, and beat cancer. It was horrendous at the time though. She needed to make the effort with me though and she did but it took time.