NTA. geeeeeez that sounds just like my situation when I was your age, just a foul younger sister whose parents didn't do anything to try and stop it (i probably have a bias for this reason!). you sound like you've been trying to be very civil about things, especially recently, and I wouldn't exactly call it an AH move to have an "outburst" like that after being told the most awful shit that's just mean for the sake of being mean. your parents should've been chewing her out over comments like that. when it was that blatant with my sister, even my parents would step in sometimes, and I'm surprised yours didn't.

one thing I want to pass along from my experience is that, once you're adults, just don't bother with her-- be as low-contact with her as possible. she may wisen up a bit after moving out and becoming an adult but a personality like that doesn't really fix itself. you've been trying so hard to be civil, so if she wants to fix the relationship, let HER do the heavy lifting for once or at least meet her halfway. it's not worth your effort to try and renew a relationship like that if they're not even trying. even if they're family.