Dear Sea,
without trying to peg your sister into a explanatory diagnosis, I did see something that I recognized in myself at the early stages of the unravelling of my denial/suppression/unconscious avoiding and the healing process. It's a thing I call "misery loves company"... and it's something that sibs learn in dysfunctional families...
if things are really outrageous - bordering on out of control (and sometimes simple life change can trigger this) - an emotional need for stability, safety, or recognition of self develops. In lots of dysfunctional families the kids' needs aren't even recognized, much less paid attention to. But instead of asking for help problem-solving, or support... the dysfunctional route of this misery pattern is to replay all the events (usually embroidered with lots of blame, attempts at mind-reading or imputing impossible to know motives) and of course, intense anger. The idea applied to sibs... is that if both sibs are on the same page - there is safety in numbers. I learned this dysfunction from my mother; saw it played out daily in a horrible work situation among the subordinates and still am on the receiving end of this - and the other things people resort to having grown up "without parents" - from my brother. It's a dysfunctional way to shunt off dealing with and owning emotions, at the core, I think. And a way to relieve the emotional "pressure" or need that builds up. Right now, my brother is on the verge of discovering his own version of my Twiggy-story and so he's almost like a different person each time I talk to him. Don't know if he'll ever get brave enough to dive into his story or not; I'm not pushing. But I do protect myself and also verbally draw lines with him when he's being unreasonable, wacked, or attacking me for no reason.
I do still find myself reaching for this "game" from time to time - but less so now. I know it doesn't satisfy my real need and when I catch myself doing it, I give myself enough "time out" to find the words to say what I really need. Having been on both sides of this - victim and perpetrator - I know how unpleasant being the target & dumping ground can be. My mom still does this on a regular basis to me and I am learning to be on guard - or detached enough - that I don't get hooked into playing the game, nor take it personally when she turns to attack me for not playing it. In my mom's reality, not playing her game equals not caring, not being loyal, not standing up for her. And on those occasions where I can stay in my own detached space... and actually suggest steps that she could take to be responsible for her own emotions and make changes externally that will satisfy her needs... that's when I piss her off the most. My mom has a lot of traits of a BPD patient; I had a lot of "learned behavior" that could be classified that way, too - but I've been able to change that and "grow out" of it. It wasn't the real me, at all - but it sure explained why I distrusted and hated myself! And it was quite "normal" for me to attempt to throw that off (it's like a hair shirt that bites, too) onto all the people around me... it was how I learned to ask for help. Didn't get me any of course!!

Now that your sister is a little calmer, it might be a good time to reaffirm your caring for her and if she accepts that, then perhaps try to explain why you can't let her dump anger/rage on you... but be careful with the second piece! You might allow more time to go by before attempting the second part. It could re-trigger her.
Hope this gives you your own insights into what might be going on! There is possibility for hope that she'll work it out on her own, too.