I’ve got a very complicated & difficult situation with my MIL and FIL. I’m posting here in case anyone has experienced anything similar and has advice to give. Sorry in advance, it’s long!
MIL and FIL are in their late 70s and 80s and have three sons, one of whom is my DH. All three of the sons are grown with young families of their own. DH and I have lived abroad for some years and, pre-covid, visited every few years.
The heart of the problem is MIL, who has a very bad combination of issues. She is a severe hoarder, to the point where her and FIL’s house became unliveable a few years ago. MIL had a fall and they moved out temporarily while she was recovering, into what was supposed to be temporary accommodation, but it became their permanent base and is now being rented by the week at considerable cost. A combination of many decades of accumulated junk, plus a leaking roof has meant that everything in the house is now covered in mould, vermin are running around, utilities have long been disconnected etc, so no toilet - they were using a bucket 
. It’s not a place anyone could live in, let alone two elderly people locked in a toxic co-dependent relationship (more on that in a moment). The state of the house is a deep source of shame for MIL, so any attempt to talk to her about it goes nowhere - she just starts screaming hysterically.
Unfortunately, MIL’s hoarding goes along with a delusion that FIL is actually incredibly wealthy and is hiding millions from her, and also has had various mistresses on whom he has lavished his money. According to MIL (who comes from a fairly wealthy family and is addicted to buying absurd lots of gifts from high-end stores to demonstrate both her bounty and her class position 
), FIL is loaded and has the ability to buy her the luxury mansion and luxury car that she deserves. Because he won’t do this, she feels justified in making his life a living hell and also letting her own house run into a state of unliveability. (This is a conscious choice on her part - DH has told me various stories, eg. how MIL would make FIL do the family’s laundry when he and his brothers were school-age, and she very deliberately broke the washing machine so he would have to wash the clothes in the bath. So all through his school days, his dad did the family’s laundry in the bath.)
The reality is very far from this. DH, his brothers and I are 100% confident that FIL has never had an affair, does not have millions stashed away and is in fact probably deep in debt from many years of MIL’s performative overspending and complete inability to understand that it makes terrible financial sense to, say, rent a high-end car by the week when you could just buy a more modest secondhand one. As the years have gone on, it’s become increasingly hard to watch his decline: as well as getting frailer, he's gotten progressively thinner and more unkempt. His clothes have become more threadbare and stained, and prior to them moving out of the house you could smell the mould and mustiness on him: it's really not the kind of state a closet millionaire or even someone who was very frugal would allow themselves to get into. It is also very obvious that MIL controls everything he does. This was the norm all through DH’s childhood, and I’ve seen it many times myself: MIL has to have control of the car keys, she controls access to FIL's phone, when she phones DH she will allow FIL only a few short minutes to talk to him and snatches the phone away when she disapproves of what he is saying, etc. Most distressingly of all, FIL seems to have lost the ability to look after himself, including eating enough: his own needs now take second place to MIL's demands, and the need to minimise the abuse he gets from her.
Here's one relatively minor incident that illustrates the difficulty of helping him and stopping her. We were at the the zoo with them and she started ordering him to buy our kids completely unnecessary and ridiculous things from the gift shop, despite our repeated forceful insistence that they didn't need these gifts and that she shouldn't be wasting money on such things. (I'm happy to have grandparents buy our kids treats and presents, but not when I know it'll cause hardship later.) I jumped up and ran after him and told him not to buy the things, and he just said quietly that it was easier to just buy the things than deal with the falllout later. Later that same day she wouldn't let him buy himself anything to eat, and of course we pressed our own snacks on him and got our kids to offer him food (thinking that she wouldn't stoop so low to get angry at small children), but she immediately got angry, jumped in to say no on his behalf, and then he refused anyway because he knew accepting would just mean more abuse later. Shortly afterwards he almost passed out from lack of food. I was absolutely infuriated and got the zoo security people involved (at this point he had to lie down and I was worried he'd need an ambulance to be called, plus she was beginning to get hysterical in front of my kids who are quite young and I didn't want them subjected to her screaming), but he recovered, insisted he was fine and MIL swept them both home.
If you are reading this and thinking ‘This sounds ridiculous: they are adults, why don't they just stop giving in to her?’, it’s because MIL is the most proficient manipulator I have ever met. She controls people with strategic screaming tantrums, and if you're able to stand up to them (as I have, once or twice) then it becomes clear that once you have walked away, she will take out her rage on FIL, so if you continue to cross her then it's him who will suffer. Normally these rages when other people aren’t around, but once I became part of the family I was privy to a few and they are genuinely unbearable - she screams and rants hysterically for literally hours on end and will appear to lose control and put herself in danger (eg. by trying to get out of a car in the middle of the road when stopped at traffic lights). Even if the screaming isn't directed at you, it’s extremely stressful to experience even a few minutes of it. FIL has this all day every day, I suspect, and quite honestly I don’t know how he’s still standing. I don’t even know how she manages it day in day out, as it must be incredibly physically taxing to scream for hours on end, but she does. (When DH was a kid the neighbours occasionally called the police because it got so bad: the police did nothing.)
The obvious solution would be for FIL to leave MIL, but he won’t. It’s a really severe example of coercive control, where he seems to have lost the ability to care for himself or go against her in any way. One of my BILs has rented him a small studio apartment where FIL can go for respite, and he does spend occasional nights there, which is something, but every time he goes away MIL gets even more convinced that he has a mistress somewhere and redoubles her abuse of him when he returns. He seems to feel responsible for her mental health issues and so can’t abandon her (which is an indication both of his integrity, and also of the iron grip she has over him).
In the past decade FIL has had a little bit of money here and there - certainly not in the order MIL thinks, but probably enough to live a modest if frugal retired life. In the past few years, though, it's become clear that his money has completely run out, as he's stopped paying for essential things and has also had to start asking his sons if they can give him money. He obviously finds this very embarrassing but the alternative is getting screamed and shouted at by MIL, so he does it and then passes the money to her (so she thinks it is his, and as he keeps producing money this feeds her delusion that he still has a pile of money stashed somewhere). DH & I would be incredibly happy to give FIL money if we thought he would use it to buy himself food, non-mildewy clothing, and other things to improve his own life, but we know the money won’t be spent on that - he’ll end up giving it to MIL so she can buy herself and others luxuries that should be much further down her priority list than, say, safe and clean accommodation. I think he probably waits until she's screamed herself out and has fallen asleep, and then sneaks into the kitchen to get scraps - he's able to get himself some food but only on her terms.
MIL clearly needs help but will not countenance any suggestion that she has any problems, and will not engage at all with any support we offer. Even a gentle, supportive conversation intended to help her inevitably results, usually within less than a minute, in her hysterical screaming (often the same thing over and over just so she doesn’t hear what we are trying to say) and her repeated insistence that FIL has plenty of money and is hiding it, giving it to his mistress. She hates the idea of taking money from her sons, even though they are all working professionals and in a much better position to fund her than her 80-year-old husband, which is why the money given to FIL which then gets funnelled to her has to be done on the sly. The amount of lies and deception is staggering.
So, that’s the background (and there's plenty more - I’ll give that detail if needed, but I’m trying to keep it as short as possible so as not to put people off reading this).
I’m posting now because after decades of deterioration, things have reached a crisis point. Whatever money was funding MIL & FIL’s stay in the pay-by-the-week expensive apartment hotel seems to have run out, and the hotel is trying to throw them out. MIL has apparently filled one of the rooms with hoarded things already, and has been verbally abusive to the hotel staff, as she eventually is to most people who get in the way of what she wants (she has had multiple restraining orders taken out against her). I suspect one or both of my BILs have been funding the hotel stay (without admitting it to their partners, but that’s another whole thread 
), but one of them has lost patience with his mother and said he won't pay any longer. I think this is actually a good thing because it may prompt change: the expensive hotel is just a band-aid solution that isn’t at all sustainable long-term. If she and FIL sold their house then they would probably have the money for a modest flat in a retirement complex, where they could get the care they need as they get older, but MIL would consider that far, far beneath her and the luxury house she imagines herself to deserve. Also, she will literally never agree to do anything with FIL - she disagrees with him about the smallest trivial decisions just on principle, so she certainly isn’t going to agree to sell the house they jointly own.
Personally I'm of the opinion that actually this crisis might be something we could take advantage of, so to speak - eg. the hotel gets the police involved and that triggers the involvement of social work and mental health services - but my DH and BILs are very uncomfortable with that (she is their mum after all, even after everything she’s done).
So, I’m posting to see if anyone has a family member or friend who exhibits some combination of these problems. If so, I would love to hear of anything that helped the situation, or even just accounts of how this has played out for other people. If the police or mental health services have become involved, it'd be good to know if that’s been positive or negative. MIL and FIL are not in the UK so I am not as familiar with all the services/laws in their country (coercive control is, for example, unfortunately not illegal there so I don’t know how/whether one could charge MIL with it), but it is an English-speaking country that is very similar to the UK in many ways.
Thank you & sorry that was so long (it was cathartic to type though!)