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

Your sister has the right to a personal life, to remarry and have a late life baby. However, that was obviously going to have a huge impact on her young adult daughter, and your sister ignored that.

The natural consequence is that her daughter, having been sidelined and effectively replaced by a shiny new relationship and baby, wants nothing to do with any of it. Her mother's choices have hurt her deeply, and that's perfectly understandable.

I know several people with 20 year sibling age gaps including my dad and his siblings. Not one of them has anything more substantial than a vague relationship and they certainly aren't siblings in anything beyond genetics. Their upbringings were too different.

I'm glad you're having your niece to stay. I think your sister was naïve not to expect this fallout from her choices.

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.

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.

Without knowing the ins and outs of the situation, I think you’re being harsh here.

She’s a young adult wanting to enjoy her life and she’s suddenly surrounded by a man she barely knows and a baby who will control the entire house. Whilst she may grow to view the baby as a sibling once it’s older and is its own independent person, but whilst it can’t do that then nice would just be a default babysitter and “extra pair of hands”, what independent 20 year old wants that?

Mum has chosen this path in life, and I have no doubt that nice would have made her feelings about the partner etc way before a baby came along. Mum made the decision to have another baby knowing this, and is now living with the consequences of that decision

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.

this.

and then the niece says "fuck that" (as indeed I would) and no one gets anything.

You’re very sweet.

Not having her to stay is equally taking her sister's side.

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.

Well you clearly have the same maturity level

Most people seem to be commenting on whether niece is right to refuse to see her mum and not OP's actual question.

OP is an adult; her niece is an adult. If OP would like to have relationship with her niece and is happy for her to stay then she should invite her to do so.

In view of the circumstances I would set a hard boundary that I would not discuss the sister/niece relationship as that was for them to resolve between themselves, with the aid of counselling if necessary.

If either the sister or the niece have an issue with this, then that is their own issue - they cannot expect OP to "take sides".

It actually doesn't matter as to the reason for niece refusing to see her mum - it's not for OP (and certainly not for randoms on a MN forum) to adjudicate whether it is "good enough" or not.

There is a balance needed to ensure DD spent time with her DM alone but otherwise her DM left a healthy gap between raising her child and meeting a new man.
When she fell in love she jumped feet first into it, agreed, my point is she could have done this through DD’s childhood but she waited until DD was older.
It looks like DM wants her to be part of the home, DD doesn’t want that. It’s not like she isn’t invited to stay and has been cut off, she’s not number one priority anymore but still important.

Ooh burn!

I think I’m more realistic about how hurt the niece is.

Is it not better if you do let niece stay with you ..as least you can talk to her and tell her mum she's safe and fed with you .
I do find it odd the mum would prefer her alone in her uni city than safe At her aunts house

End of rhe day your sister failed to put her child
first, insiting her partner came everytime she did something with her daughter was incredibly selfish. Maybe your sister needs to actually think about what she’s done in this and think of a solution.

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.

From the Op

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 would definitely prioritise your niece here. It sounds like your family are basically her only family now and she needs someone to love her unconditionally. I am absolutely shocked that your sister would tell you not to let her stay with you - how on earth could she possibly imagine that would help anything? And even saying that to you is going to further damage her relationship with her daughter.

I wonder if her partner is controlling. How quickly she had him there and as much as she did fits the pattern (love bombing, I want to spend all my time with you, read: I want to keep an eye on you and make sure there is no time for you to be secretly seeing anyone else, texting anyone else, saying anything to anyone that I’m not aware of), same for the never letting them spend time together on their own.

It’s also not at all good that she prioritised him, as you have already said, and entirely fair that her DD doesn’t want to see her now. Her mum might as well have told her to f* off basically with everything she wanted and needed from her mum, now she’s showing her mum this is what happens.

OP, I think you're putting yourself in the middle of this by letting her stay at yours and your relationship with your sister will be negatively affected. If she stayed at her university city this would stay just between them and you could interact with both freely. Having her stay at yours when she has cut her mum and half sibling out her life is taking a side.

You niece is being very immature but that's perfectly normal for a 20yo. 20 is legally an adult but really barely out of puberty and the brain doesn't finish growing to adult configuration until about age 25-27. She can be forgiven for her silly attitude which she will probably manage to reconcile by the time she is 30, hopefully sooner.

Yanbu. You are family and you love your niece. It is perfectly sensible to give her some loving nonjugemental space in which she can focus on her education and her own ambitions. Sharing a house with a newborn baby isn't a good way to spend university holidays even if there wasn't a rift.

Your sister is being unreasonable. Her daughter can't be shamed or pressured into changing her mind and trying to make it more practically difficult to avoid coming home will not help.

No
She's not a nice person,turning her back on her baby sibling is actually quite nasty .
I understand not wanting a step dad ,not wanting him moving in ,Anne being upset at 17 .
But 20 is an adult,and she's causing hurt to her mum and ..that's not a nice person
Baby is here now and niece needs to grow up and accept the situation

Thank you for being such a good auntie. I didn’t have a lot of safe adults growing up so I think I’d appreciate being cared for like this. Having someone kind and steady having your back is very powerful, especially in early 20s. I always try to prioritise kind and responsible young people. We need more of them.

I think where going to have to agree to disagree as you don't seem to understand what the word sibling means

It's ok to say
"Your daughter is fine, or gone back to uni", or whatever.

I didn't say personal stuff, to which I agree should be with consent. But the spirit of my message is that it's good for her to tell her sister that the daughter is doing ok. Better than nobody knowing where she is and how she is.