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

She doesn't sound spoiled to me. Just someone who for whatever reason doesn't feel at home in her mother's house anymore .
The sister sounds selfish to me and lacking in understanding.
Her daughter doesn't want to live in a house with her mother's new partner or child and she feels more at home with her aunt and her cousins who are more her age. I would be grateful to my sister for caring for my daughter.

Of course it is. If niece had said "Can i come and stay with you because it's closer to my summer job/it's on a better bus route/I don't want to wake the baby up coming in late after socialising/it's a quieter environment to study - but I'll of course still be going over to see my mum and sibling all the time and going on days out with them" then OP would not be starting a thread and niece would just move in with OP.

That's the comparator.

not relevant. The whole point is that the OP should not be judging or trying to make decisions based on might haves or what ifs. The niece has made her decisions. From what the OP has told us she has made her own life and choices.

I think that’s exactly how my niece sees it

My sister thinks it’s fine as she waited til my niece was over 16 to move him in and that my niece was starting to live her own life, was out with friends a lot and was planning on leaving for university soon. She didn’t see why it made a difference that her partner tagged along when they did things, My sisters partner apparently paid for all the ivf costs so my sister says not doing that wouldn’t mean she could help with university costs.

My niece told me that she feels like she hasn’t had a mum for years and she has dealt with it, moved on and made a life for herself. She says her mum is only bothered now because she’ll feel embarrassed that she is staying with us, but didn’t have a big problem with her not having much contact when she was staying in her uni city for holidays.

Your niece sounds incredibly spoiled and I feel for your sister, I wouldn't stop her staying though as at least it keeps communication open.

I think that’s exactly how my niece sees it

My sister thinks it’s fine as she waited til my niece was over 16 to move him in and that my niece was starting to live her own life, was out with friends a lot and was planning on leaving for university soon. She didn’t see why it made a difference that her partner tagged along when they did things, My sisters partner apparently paid for all the ivf costs so my sister says not doing that wouldn’t mean she could help with university costs.

My niece told me that she feels like she hasn’t had a mum for years and she has dealt with it, moved on and made a life for herself. She says her mum is only bothered now because she’ll feel embarrassed that she is staying with us, but didn’t have a big problem with her not having much contact when she was staying in her uni city for holidays.

All of that might be true, but the issue seems to be since her mum had the baby - which is a different matter altogether. I would let her stay but saying that she wants nothing to do with her mum because she's had a baby and wants nothing to do with the baby either, makes her sound spoilt. She's an adult, your sister is entitled to a life of her own. Maybe she moved too fast at the beginning but that apparently has nothing to do with her behaviour now.

Is there a chance the niece has exaggerated how her mum never had time for her? She may not have but you never know. Teens can be manipulative unfortunately. I have a daughter now the same age who moved out at 18. When I started work she started saying I never had time for her due to my work commitments, she even went as far as crying begging me to not go to work to the point I'd have to call out because she would have supposed panic attacks but then as soon as I'd be off work she'd sit in her room call her boyfriend invite him over then act like i wasn't even there. She moved out telling everyone I was the problem yet we would spend all the younger kids summer holidays dancing to her tune, she would refuse to come on days out yet didn't want to stay home without me there which would prevent the other kids from going out places. She told my sister I kept her prisoner in our home said I wouldn't allow her to go out yet the simplest thing like taking my kids to appointments she made difficult by kicking off and saying I couldn't go without her and I'd have to rearrange them due to her refusal to leave the house. Her mum shouldn't be telling the aunt not to have her round though the girl is an adult and can stay where she chooses.

You absolutely made the right decision. Both parties are grown adults free to live their lives as they choose. Your sister is thinking emotionally and from her perspective. In truth, if you were to push your niece away and in effect distance her from wider family too who knows what that would do to her mental health.

No he didn’t. He moved in 3 years ago and they were together 15 months before that. It was the first man the mother introduced to DD despite having had other secret relationships.

She fell for this man hard, he fell hard too and he paid for IVF.

Maybe mam deserves love too.

I don’t agree with her not cutting out special time to spend with DD, taken on an extra job to save for her uni but finding a loving relationship is not the problem.

Nor do I agree with moving a new partner in within a year or 3 years from leaving the previous relationship. This lady stayed single for 9 years.

It sounds like DN was pushed aside in favour of new bloke. DSIS has created this situation so she has to live with the consequences.

She isn’t at all. When my sister met her partner, she introduced him very quickly to my niece. He was staying over most nights within weeks so almost living there and then properly moved in after about 15 months. My niece would ask to do things with my sister and she always invited her partner along. My niece feels like she put him first and that as soon as she met him, she no longer spent time with just her. I spoke to my sister about it at the time but she told me my niece was fine and it wasn’t my business. I couldn’t say much because what I knew was through my daughter telling me what my niece was telling her. My niece started having more sleepovers with us but my sister said that was just what teens did with their cousins and not to do with her being upset with the new partner living with them.

So your Dsis had relationships when DD was younger, 7 to 15 and never introduced any of these men, she waited until DD was 16 to move her partner in after 15 months, it wasn’t not the best decision but it was far from the worst decision.

Plenty of parents move on within a year, moving a man in and having a few children, in terms of single parents who are still looking for love, I think her DM was fair.

Why has she no money to support DD if she is a high achieving person? Has she had a job over the years? Can she access credit? Who pays for uni ATM I don’t understand that part.

What a brat
What teen decides to ignore her sibling because she didnt like her mothers partner moving in
What a brat and a selfish one...yip all good behaviour until it becomes familair..she needs to grow up

I disagree. Having a friendly ear to talk to is sometimes much needed. I think refusing to talk at all about the issue may be a bit harsh. But the OP shouldn't take sides, that's different.

Your niece sounds incredibly spoiled and I feel for your sister, I wouldn't stop her staying though as at least it keeps communication open.

Not really, if she doesn't normally stay status quo is maintained. Having her stay over the holidays is new and actively choosing a side. Not judging picking a side if you believe the sister is in the wrong, but it's not a neutral action.

So when she was literally asking to chat to her mum and spend one-on-one time with her and was completely ignored because her mother was prioritising her brand new bloke, that was fine?

But now she’s choosing to ignore her mum’s requests to chat she’s being unreasonable?

She’s obviously deeply hurt that her mother sidelined her in favour of a man, and now she’s grappling with the bizarre situation of having a step-sibling 20 years her junior, which must be both painful and unsettling.

Every poster describing her as ‘spoilt’, ‘bratty’ and ‘attention-seeking’ is coming off like someone who’s done the same to their own children. Or is entirely lacking in empathy and the most basic understanding of human psychology.

The mother - the adult - is in the wrong here, not the child. And it’s on her to take responsibility for how she made her daughter feel, then and now, and to make whatever concessions are needed to rebuild their relationship.

Just a suggestion, could you suggest that they both write a letter to each other. Put down their feelings on paper. And then read what each other has to say. Sometimes, meeting in person and having that conversation face to face is hard.
From what you’ve said, I’m on team niece, but she might regret cutting her mother (and sibling) off, without a backwards glance, at some point in the future.