My SIL has serious mental health and personality problems. She's quite dangerous. I suspect BPD/EUPD but I don't think she's diagnosed, mostly because she lies a lot to health professionals and as soon as she gets a hint they suspect EUPD she never sees that professional again. I say I think because she lies a lot to me as well- she said she had been diagnosed with ADHD, ASD, BPD and GAD and was being medicated for all of them, then she said that actually her then-boyfriend (not a HCP) had "diagnosed" all of them and bought drugs off the dark web to "treat" them. The she accused him of drugging her (even though she participated willingly in all of this), managed to get into a women's refuge and they moved her away. She says she's done all this to protect her son, but she's left her son with his Dad and not seen him for months.
She told me months ago that she no longer wanted her son "ruining" her life, she wished she'd had an abortion, and she had a plan to leave the area and get a nicer house with a dog. She's extremely skilful at manipulation and has successfully manipulated the domestic abuse charity she's with to make all this happen for her.
Anyway, I digress, but over the 10 years I've known her she has been absolute chaos and attempts to help her have always spectacularly backfired. I don't like her, but I do feel sympathy for her because of the things which made her the way she is. Having said that, she's nearly 50 now and is still using her childhood as the excuse for everything.
My husband said he was done with her a long time ago and I've tried to maintain contact because I don't want him to lose his sibling, even though I'm her main target for vitriol when she goes off. I have stopped now and stepped back after some particularly bad behaviour at the start of this year which was just too draining for us. She occasionally sends me a message, usually full of flattery and sweetness and light, which I either ignore or remind her why we're not in contact. Can't bring myself to block her completely though.