We are in the USA. My MIL lives alone in a 3 bed bungalow rental in a gated community with a lake, pool, gym, restaurant etc. It's not for 55+ and my SIL moved there first with her 2 girls about 10 years ago, when prices were still reasonable. We helped her buy her house as her husband left her.
My SIL is lovely and has always done whatever she can to help her mum and dad (who are divorced but her dad is remarried to a lady 25+ years younger, so he has more practical help). My SIL has made so many sacrifices for her mum over the years (kind of raised to do that).
About 3 months ago my MIL accused her SIL's eldest dd aged 20 of sexually abusing her sister age 14. Both girls swear this never happened and noone on the family believe it happened either. (TBH one of the reasons we never lived near my MIL was because I was sure our marriage wouldn't survive it, as she was always accusing me of bizarre things - although not sexual abuse, thank God!).
My MIL has form for hallucinations and accusing all sorts of people of sexual abuse. My MIL told my DH she was going to call CPS (Child Protective Services) and I told him to warn his sister. She immediately cut off all contact with her mum and also wrote a letter to the judge to a case which my MIL was involved in, describing her history of hallucinations and that she shouldn't be around children, as she was suing to get access to a granddaughter of one of her other kids. My MIL lost the case.
Now the whole family have (once again) begged my MIL to get psychiatric help but she refuses and swears up and down this is something she witnessed in broad daylight in their living room.
So now her twin sister and her dd (my SIL) refuse to see her. My DH is the only one in contact with her. He tries to monitor her. It seems to me she could be developing dementia, although she has been having hallucinations for decades and always refused all medical help. She also has never wanted to be in any kind of retirement community, we help her financially as she has always blown through any money and has nothing but debt.
I know we are in another country and another system (she is in California). But does anyone have thoughts on how to proceed as she has become very socially isolated?