Please offer your advice, I’m stuck.

So I have an older sister who is constantly starting fights within my family. Whether she’s fighting with me, my mom, dad brothers etc it’s always someone and usually it’s me or my mom. So she’s been like this as long as i can remember, even when I was young she would pick fights with me (she’s 10 years older) so she’d be like 20 arguing and being mean to a 10 year old. She continued this throughout my teenage years. I always would argue back and she would say mean things but we would always end up making up. Now I’m 22 and I’ve had enough. She started a fight with me in October over walking faster than her at the zoo. I acknowledged Ger feelings and apologized up and down. She wouldn’t let the argument go. I have severe anxiety and told her I was off my meds and not going to argue with her about this because I already apologized over and over. She told me I wasn’t her family, fuck you etc. I blocked her. I ended up unblocking her a week later because she told my dad how upset she was etc. so I see her at my thanksgiving etc. she acts fine towards me, like nothings wrong. I even reached out to her asking about my niece and she responded once, then ignored me and went and talked shit about me to my mom. I let it go and gave her another shot. Seen her at Christmas she acted completely fine towards me. I even offered to get together and decorate her daughters dollhouse. Then afterwards started talking shit about me and my fiancé to my mom, and apparently does it all the time. My fiancé has never done anything to her, he doesn’t deserve that at all. She makes up lies to try to pin my mom against me. She makes up arguments all the time. She acts totally fine in person then goes and talks badly about us. She’s toxic and I’ve had enough. But my niece. I don’t want to go without seeing or talking to my niece but I don’t want to see her or talk to her ever. She makes my anxiety and stress so high. I don’t know what to do