Im sorry your dealing with this type of situation.

Of course no one here is a Doctor so unable to diagnose. Just my own opinion of course.

Are her daughters grown and away from that living situation? If not there safe being needs to be the first issue. Are they getting any help ? Therapy?

She could have any number of different illness's... Usually Bipolar runs in cycles of weeks or months if shes changing over a few days maybe its bpd (borderline personality disorder) Does she drink or use any type of drugs? Lots of things can cause moods to shift.

Theres are just guesses based on your Post .

How to get her to seek help ? You can catch her in a good mood and bring it up in a loving manner . It's up to her to get help and only see can reach out for it.. No one can force anyone to get anyway. Sad but true.

I wish you the best