How can I reconcile my feelings about my bpd sister after going NC?

First of all, I'm new to this community so apologies if this isn't allowed/the flair is wrong. I was searching for somewhere to go where I can get some advice from people who will understand my position, and I saw this sub mentioned in r/raisedbynarcissists

TL;DR: cut off bpd sister in January. Was doing relatively ok, all things considered. Sister contacted me yesterday and brought back all of the negative emotions from before. I miss her but I can't be around her anymore, how to cope between now and next therapy session?

Some background, I'm 26F and my younger sister(23) has bpd. Last year my grandfather died and, while she had always been bad before, I think his death kicked her disorder into overdrive. I don't want to go into details right now, but lately I've been the main target for her... I don't know what to call it. Rage? After escalating incidents all through 2019, everything came to a head in January 2020 when she accused me out of the blue of something truly evil that 1) I never did, have never wanted to do, and never would do, and 2) could get me fired from my job working with children, should she feel the need to go public with this fucked up lie. I don't know if she intentionally made it up, or if she truly believes it either, which scares me even more.

For my own safety I decided to cut her off entirely. I started therapy, too, but since that's still fairly new I guess it'll take a while for any positive effects? It's March now, and I thought I was doing better, but yesterday my sister emailed me out of nowhere with a blame-shifting non-apology (think "I'm sorry your feelings were hurt, but you were being mean and deserved it"). The message was not particularly long, but it just filled me with so much anger and all of my emotions from the incident in January immediately came flooding back. I responded coolly, saying I needed space basically, but inside I'm still so torn up, and angry, and hurt, and sad, and all sorts of emotions and I don't know what to do. I won't have another therapy session for another two weeks (only monthly, due to finances) and I was hoping for some advice on ways to cope in the meantime.

I didn't sleep all last night because my head was so full of thoughts about my sister, both angry and longing to return things to normal. I adored her, despite the shitty things she did to me in 2019, until January. What she said rocked me to my core and turned everything upside down. And on top of all that, to realize the reason she hates me so much is based on a LIE...

My parents are no help. My father repeatedly asks "Where I am with my sister" and "am I still ignoring her," while my mom gives me unsolicited updates on how she's doing. It hurts because it makes me miss her so much more, and they don't seem to even realize or acknowledge just how serious I am about wanting her out of my life. And to complicate things, the grief I feel at losing my sister feels the same as the grief I felt when my grandfather died. Everything sucks and no one understands because, "if you're so upset, why don't you just apologize to each other and move on?"

Sorry for the word vomit. Do you have any tips for coping with the irrational grief and anger that comes with going NC?