Let start with:
I have a very difficult relationship with my sister, it's not that she's a bad person, she's just incredibly selfish and self-involved. Everything has to be about her, everything revolves around her.
She has a good job and her husband has a very good job, they're extremely comfortable and have a lovely life (on the surface)
They have two girl, my nieces, and my eldest niece is the apple of my eye, I absolutely adore her and have always been extremely close to her, since she was born (which I was there for)
Me and my sister, up until recently, had a very close relationship, but I'm now approaching middle age, and I'm not as tolerant of the things she does anymore. Example, we plan lunch, I book a table, I will get there and be waiting 40 odd minutes for her to turn up. My birthday, I want to go to a certain restaurant, she doesn't like it so she won't come, so I have to change my entire day to suit her (and my parents will back her, infuriating me even more) and if I don't agree, it's made very clear that IM the a-hole for not changing my plans.
Recently, my niece shared with me how unhappy she is at home, how she doesn't feel listened to, she's ignored and she said very clearly, there is a very big difference in how her parents treat her vs. Her younger sister. I tried to reassure her, but then my dad (grandad) gave my sister some money to split between my nieces. He told my niece, and she said thank you. She went home, saw the money on the side and said, who's is that? My sister said, it's not yours it's your sisters. My niece stood her ground and said grandad had said it was to share, my sister didn't know what to say, but it's the tip of the iceberg, there are numerous incidence like this. She told my niece she's an entitled little c*nt, 'that I hate people like you always expecting everything' she can be extremely cruel (and I think she's starting menopause as well, explains a lot of the erratic mood swings but has made her entitled behaviour a million times worse)
I finally felt I had to say something, and my sister didn't like it and I know it's not nice to hear that your child is so unhappy but I was trying to stop their relationship from deteriorating even further. Her response, to ban my niece from seeing or speaking to me, which isn't going to work and obviously caused my niece to be even more upset. I'm often visiting my parents and my niece is down there occasionally and so i see her there, and also text most days and I know my sister is jealous of our bond, but she never has the time to talk to her, or listen to her. If my niece has a problem she needs help with, she always rings or texts me because she knows I'll respond straight away and I'm never too busy for her. Exactly the same as I was for her mum as well, but that was always a one sided relationship anyway.
My nieces birthday is coming up, and she said she wants to do an escape room, and i said I'll get it for your birthday and she asked if I'd go with her and her friends, of course I said yes (but I knew my sister wouldn't let that happen)
Obviously she's gone home and told her parents and friends be had a snotty message from her dad, as her mum still won't speak to me, and to be honest, that's fine with me, but he's basically said my sister is going with her, thanks but no thanks.
I mean, my sister is always far too busy to do things with my niece, she's happy to do things with her youngest, but she calls her eldest difficult and moody, but, unfortunately, teenagers that dont feel listened to, don't want to communicate. I mean she told me first when she had a boyfriend, and I said, you really should tell your parents 'why? They won't care' I don't want my niece to feel she has no one to speak to.
Also to add, I have depression many years ago, and that was thrown into the argument with my niece about me, her dad said to my niece, you shouldn't really take advice from your auntie, shes got depression and her advice is very poor. Honestly what I'm up against.
Please give me so helpful advice, because I'm at my wits end with this.