Hi, just looking for some outside perspective and advice really.
My sister is 26 & lives at home with my mum. She is the most negative person imaginable. She is angry, aggressive, shouts, moans and generally talks to my mum (and often me) like absolute shit. An example of how my sister speaks to me like shit is this phone call back in November -
Me - Hi, do you know if x,y and z have confirmed for Christmas dinner?
Sister - It's you organising it, how the fuck should I know?
Me - Oh, just because you've been with them today, I thought they may have mentioned it?
Sister - Oh for fuck's sake, I can't be arsed talking about this, I'm not interested who comes, I don't even wanna fucking go with you's. Ask them yourself puts phone down
This is a genuine conversation we had and I started crying when she put the phone down. She's horrible to me.
She shows no interest in me or my daughter and it's hurtful. I have secondary infertility and can't complete my family, so this is the only niece she'll ever have. My daughter (almost 8) idolises her and doesn't understand why my sister rarely wants to spend time with her and why she has no patience with her. This really hurts me.
I have bit my tongue & let her speak to me and my mum like shit for years, simply because I worry if I retaliate then the damage would be irreparable. My mum lets my sister walk all over her.
Things came to a head at a family party last week. Long story short my sister started swearing loudly at my mum for the most ridiculous reason. She was making a scene and it was embarrassing. I told her not to speak to my mum that way and that i'd had enough of her. She told me to "shut the fuck up, hardknock, who do you think you're talking to?" I lost my temper with her and told her i'd smack her in the face if she uttered another word
I told her I wouldn't be enabling her behaviour any more by keeping my mouth shut and I told her to leave the party before things got out of hand. She told me she was finished with me and that I wasn't getting the money back she owed me. That is how childish she is 
I like to think i'm a good sister. I love her very much, drop things no questions asked for her, lend her money, help her in any way needed but I get so little back in love & support from her. She has a boyfriend who is similar to her in his attitude and I believe this is because they smoke cannabis, yet i'm not prepared to accept that the cannabis is the only reason she's so angry & horrible. I think it's just who she is?
WIBU to have nothing else to do with her? I just worry for my daughter. I can't give her a sibling, and I worry she'll be lonely without much family around her and she loves my sister a lot. If you are nc with siblings does it make things really difficult at family occasions etc?