Oh gosh friend, what a mess.
She sounds like a toxic person, and that is long term. Unfortunately, very few people that age, in that position, change.
I think at this point the only plan you can have in place is what happens when her treatments really are not working. A long term plan for you to be able to not have this foisted onto you and your family.
Ugh. I can only tell you I am sorry and I send you HUGE hugs!