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

I think you’re doing the right thing letting your niece stay with you, I’d remain neutral, your niece us an adult she will make her own decisions about her mum, I’d stop talking about the situation with your sister and your niece, unless your niece brings it up

When I was 17, my mum did something very similar and it was awful. I think you need to be there for your niece, my aunt was for me and it made all the difference.
Your sister needs to respect that you will always be there for your niece. I wouldn’t try forcing anything between them, it won’t be solved by her being forced to play happy families with them. I think it’s your sister that needs to be the one with the olive branch here.

Tbh. I am team niece. It seems like her mum forced her BF onto her daughter quite quickly even if he didn’t live with them. Her daughter obviously feels pushed out . Seems like mum chose her partner over her daughter and will need to face the consequences. Tbh Im appalled your sister wanted you to abandon your niece and refuse to let her stay. Not a good look Mummy Dearest

I’m not sure why you think your sister and her partner needing fertility treatments to have a child is relevant? It’s entirely their business if they want a baby and banging on about how they conceived seems odd.
Do you have a relationship with your new bride or nephew?

Spoiled?

Your sister and your niece need to sit down together and be open and honest with each other.
Your niece is telling you things about her mum that she is unhappy about and asking you not to tell and you are abiding by this, yet at the same time, you say you are being neutral. That isnt being neutral, although I'm not sure it is possible to stay neutral anyway.
I'd be interested to hear yours and your sisters side of things. What kind of person is your sister? Does she generally take responsibility for her choices? How long was it just your niece and her mum for? Does your sister have a habit of being in and out of relationships? What are your thoughts on your sisters relationship? Do you get along with your sister most of the time or has life taken you down different paths? Do you believe your sisters new partner is a good man?

How old is your sister?

Your sister probably wants an on call babysitter.

So your sister met her partner 5 years ago waited 15 months to move in with him and 5 years to have a child and her adult daughter isn’t speaking to her because she wanted more 1:1 time at 17? I think the niece is being ridiculous and immature, but I definitely think you should keep her for the summer and be a safe place. Some people take longer to grow up. U would say we all need an aunty who will offer shelter in a storm even if it’s of our own making.

Why on earth should your niece have anything to do with your sister? It sounds as though she has healthy boundaries.

Her mum meets a man and basically moves him in to the daughter’s home most nights.

Her mum makes no time for her, won’t do anything 1:1 with her teenage daughter but prioritises the new man instead.

Her mum then moves new man in full time, can’t even wait a year until daughter leaves for uni.

Her mum refuses to assist with uni costs in the basis of no money, but is able to find money to finance IVF.

Basically, your sister is a shit mum and your niece is sensible to go no contact.

Your poor niece. She is lucky to have you

Of course let her stay. She's an adult, it's up to her who she sees and her mother can't force her to do anything.

She shouldn't have moved her squeeze in with her 17 year old daughter if she didn't want to upset her. Sounds like she loathes him - I wonder why?

Why is your niece so upset about your sister having another baby?
Has your niece told her mum how she feels?
Have you told your sister why your niece is so upset with her?

"They had a good relationship until her mums partner moved in."

"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."

"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."

I'm Team Niece, to be honest. I've never had much time for women who drop their friends as soon as they have a boyfriend, so I'm not likely to think much of a woman who drops their daughter in the same manner, am I?

when I got divorced, my kids were at uni. Neither of them ever went back to the family home which their dad kept. Within a year he was remarried and his new wife was pregnant.

they don’t see it as their home anymore. It’s got a new family and new kids living in it and there’s no emotional space for them (no physical space either!)

one of mine stayed moved out and once he rented in his second year never left and has gone from rented house to rented house.

i feel quite bad about this and have kept him a room in my new house but I’ve had to buy a fair bit away from where they grew up and honestly he doesn’t know anyone where I am. He has loads of friends now in London (where he went to uni).

realistically your niece will not spend time with her mum if you ask her to leave. She’ll just go back to her uni city and get on with her life.

Her sister, the parent in this situation so the one who should be more mature, is trying to get her daughters aunt to cut her out. I don't have any sympathy for the sister. She is being very cruel.

I think it's a good thing to give your niece a safe place to stay without her mom's partner around. Her mom picked her partner over her existing child and this is the fallout of that choice, her daughter wants nothing to do with her and her new family.