NAMI is a great resource and source of support for you, OP, your tribe is there. The short Family to Family course is also time really well spent.
https://www.nami.org/Home - Right now the home screen is about October as Suicide Prevention Month, ironically.
She sounds borderline & or narcissistic and those mental illnesses don't really resolve ime. If you have contact info for her therapist, you can reach out, they can't share info with you but can listen. Nothing you say is going to cause her to change. Is it possible that she may have early symptoms of dementia?
If you fear for her safety you can call police and request a wellness check.
It's really hard, you need to figure out what you can live with. Some people do follow through re: their threats, I personally had to make peace with that and where my boundaries are and what steps/protocol I was willing to take. Wishing you peace, OP.