I (23f) fucking hate my sister (38f) and I don’t know how to handle it

TLDR; my half sister, who I’ve never had a good relationship with, treats my parents/her parent really shittily and it’s making me so fucking angry at her. I don’t know how to handle it without possibly losing my relationship with my nieces.

Half sister technically, we have the same mom and different dads. As you can tell from the ages, she was basically out of the house by the time I was born and our mom had remarried to my dad, but I swear on my life she’s never gotten over it (25 years later). She never sought me out to start a conversation growing up – the first and one of the only times she did, I was 14 and so shocked that I barely managed to respond. I can count the number of questions she’s asked me about myself and my life on one hand. We did grow up fairly far away from each other (my parents moved across the country after I was born), but she visited a decent bit, at least a few weeks a year. Despite that, I’d genuinely say that I have no relationship with her.

I was upset about this when I was younger, but I have no problem with it now (to the point where if she were to reach out with an olive branch, I’d turn her down). For many reasons, she’s not someone I want to spend time with. The only reason I do is because of my nieces (3f and 6f) who I love dearly. The fact that I spent a lot of time with them actually seemed to soften her a bit towards me; the handful of questions she’s asked me all came after I was 18, when her oldest was born and I made the effort to visit and spend time with the baby.

So all would be ok, but… I can’t stand the way she treats my parents (including HER parent). Like it’s always been bad but this year, it’s been especially awful. My parents now live near my sister (and me) and she just treats them like shit. My dad, not even related to her, has always made an effort to do FaceTimes with the kids and buy them presents and help clean up in their kitchen if we’re invited to dinner (which they often make us bring our OWN FOOD to) and offered to watch the kids for date nights, even taken the night shift on visits shortly after each kid was born so my sister could sleep – none of that is ever recognized with as much as a thank you. This year, he’s finally pulling back (aka not reaching out as much) and maybe that’s why the relationship is degenerating, because she senses that.

And the way she treats my/our mom breaks my heart. She just turned 70 and my mom, my dad, and I planned a special trip for it. My sister never reached out to any of us asking about birthday plans and called her at 7pm on her birthday. No card. Just a 20-minute call. For her mom’s 70th birthday. Oh and to be clear, she asks for and accepts every offer of help she’s ever gotten from my parents, from taking the trash out to watching the kids to giving her $20k for her wedding, so I really don’t need to hear people theorizing in the comments about how my parents are actually abusive pieces of shit. She’s very happy to come around when she is getting something out of it, but she has never shown up for them.

I don’t give a crap about how my sister treats me, but I cannot. Fucking. Stand the way she treats my family, and having to watch them get their hopes up every time that their jerk of a daughter/stepdaughter will for once actually be a decent fucking human being and get let down over and over because she’s just not. The last time I was at her house playing with my nieces, I couldn’t even meet her eyes, I was so angry.

I’m so tired of living with this anger and how it makes me not want to visit my nieces. If it wasn’t for them, I’d walk away, but I really do want to be there for them and I know they love me, I don’t want to leave them because of their mother. I know communication is the answer but it’s not going to happen (my mom and her are both horrible at it, and I legitimately have no relationship with her beyond hanging with her kids sometimes so I’m not an option either). I’ve talked about the situation to so many friends and beyond agreeing that she’s a jerk, they have no advice.

So here I am, hoping that someone here has a take I’ve never heard before or advice that might help me deal with this shitty situation. Anything at all would help!!