I have to confess, OP, when I read your post, I did wonder if it was one of mine...
I also have a daughter of 23, who behaves like the family's Diva 24/7, who hasn't spoken to her stepfather since she was 10 (he and I split when she was 12, and she was partially to blame for that as far as he's concerned), and I have a son whose entire life has been punctuated by whether or not his sister's in a good mood. Three years ago, though, my daughter was diagnosed with NPD, bipolar2 and borderline ADHD, and she is now on anti-psychotics for the forseeable future. Our home life hasn't improved greatly - particularly as she still lives at home, and her boyfriend ran away to the other end of the country - but at least we know why she is, as she is, as she is.
However.
I won't tolerate her speaking to me the way you say that your daughter speaks to you. At all. Irrespective of why she felt the need to behave like that towards you, she needs to understand that she no longer lives with you. She lives with her "gormless" boyfriend (who is, actually, probably quite nice - she obviously thinks so!), and is an adult. Your house? Your rules.
Stop buying her cars. Stop paying her insurance. Encourage her to realise that she's been an adult since the age of 18 and that equals standing on her own two feet every now and then. My daughter still lives at home, has recognised MH issues, yet works and has not only paid for her own driving lessons, paid rent every month, but also bought her own car and is paying off a (black box) insurance debt. She also runs her little brother to and from school when I can't.
It can be done.
Yes; I get that you were worried about your son - but surely if he'd been there, he would have said something to you, and not to his sister?! It's the boy who "blew his brains out" and his family whom I feel sorry for, OP, actually - not necessarily you and your spoiled offspring. Stop mollycoddling her and let her flourish as the adult she wants, desperately, to be.
And just be grateful she doesn't have a child. Because when I was her age? My daughter was almost 3 years old...