Thank you OP for the update and I’m happy you and your mom are getting some helpful answers here. I don’t really have advice, but just wanted to say I know where you’re coming from. My sister sounds a lot like yours, except that she is single and doesn’t have kids (for which I’m grateful). I’ve been thinking it was narcissistic personality disorder but I’ll look into BPD as well. Will also read the Walking on Eggshells book.
My problem is that no matter how horrible she is to me, I can’t completely cut her off. She no longer speaks to our brother and wavers between tolerable and unbearable for the rest of us. Unfortunately our mom enables her and laughs when I suggest therapy so she can learn how to navigate the relationship.

OP with a bit of an update. Haven't heard from my sister since the incident, but told my mom about what happened, and she wanted to reach out. I discouraged her, and sent her a link to this post so she could read the replies. She was a bit defensive, as one might expect ("I do NOT have dependence disorder!") but said the advice was very thought-provoking. Meanwhile, I went looking for Stop Walking on Eggshells, and put a hold for it at the library.

This evening my mom texted me with screenshots from a book, describing BPD characteristics. I didn't see the message until later, so she ended up sending me about 10 screenshots, each with a comment like "omg this is her!!" or "they should have your sister's picture here!!" When I eventually talked to her, I asked what book she was looking through, and she said "The one that internet group suggested." She went on about how so far it was so helpful in just helping her understand that it's very very likely my sister has this mental illness, and however we react, it's worthwhile to be validated that my sister's behavior is not that of a typical person.

Things are at a bit of a standstill, I guess, since my sister is not contacting either one of us and I'm for certain not reaching out to her, nor do we have any upcoming family milestones to facilitate communication, or non-communication. But posting about it here and reading your replies was very, very helpful for me--more helpful than I realized it would be. I didn't think I'd be able to get through to my mom either, but apparently internet strangers have some kind of magic authority. So, thank you to everyone who responded. It was more helpful than you know.