MiL can't/ won't make financial or physical decisions in her best interest. We have no idea what to do.

My MiL suffers from a rare genetic disorder that impairs her mobility and cognitive functions (think Parkinson's plus Alzheimer's), the degree of damage for either depends on the individual. She walks with chorea - constant movement - but we're more afraid for her cognitive decline: she can't or won't make financial decisions in her best interest. It's putting an enormous strain on my BiL's family, as she is bouncing around between them and her house because my husband and I are 300 miles away. I know this isn't an uncommon experience, but we're feeling so lost. Every time my husband and BiL are in a room with her, they get beat down and despair within an hour.

Some details:
- MiL's son has access to her bank account. He "lives with her," which really means he shares a space with her. He's not there all the time and the most he's done for her is drive her to the hospital once. When asked about her home, she says she lives with her son, even though he doesn't take care of her.
- BiL is using MiL's money for his own purchases. He occasionally goes to the grocery store, but comparatively speaking the money he uses is mostly for himself.
- MiL allows him to steal from her under the guise of "he'll pay her back." We estimate he's stolen thousands of dollars from her since their dad died a year and a half ago. When we tell her that he is definitely not going to pay her back, she won't hear anything of it.
- Because nobody has kept up with her bank accounts, it's fallen on everybody's shoulders to pay for her medication, food, groceries, etc., which nobody can do for very long. This has been a massive burden on my in-law's family.
- She also can't or won't make decisions in her best physical interest: recently she has been in and out of the hospital because of a spat of diarrhea. It has been a continuous cycle of: her coming home -> her not drinking water because going to the bathroom takes too long for her -> her being constipated because she won't drink water -> taking a stool softener for the medication -> having diarrhea and going to the hospital for dehydration. She's been in and out of the hospital for months because of this.
- She cannot be reasoned with on any of the previous points. I could wave the transactions until I'm blue in the face and it wouldn't matter. If somebody points out that they're glad she's drinking something, she'll put the glass down and refuse to drink more.
- My husband and I live three hundred miles away, in a second-story, two bedroom apartment, in a city a fraction of the size of the one she's currently in. I'm terrified of her coming down here to stay with us because 1) the recent spate of hospital visits means there is a pretty good chance if she were to come to us, she would end up in the hospital, in another state; 2) us living in a second-floor apartment terrifies me when an elderly person is coming to stay with us; 3) our city is relatively small, with a fraction of the options of the city she's currently living in - what little help she has up there would be almost nil here; and 4) there is no telling what her hospital stays will lead to, and whether she would stay with us permanently. Her entire family lives within two hours of her, and even if they're unhelpful at all, she still wants to be near them.

I don't know what to do. Every time my husband and BiL are in a room with her, any kind of talk about assisted living devolves within an hour. She can't take care of herself and she can't admit that. She has a home, but the boys are terrified of any kind of healthcare worker seeing it and calling DFACS because it's in such a state of disrepair.