MIL's expectations about granny flat

[RhinestoneCowgirl]

DH and I have recently returned from a visit to MIL's home city, we live 200 miles away and stayed with DH's younger brother and wife as MIL is still not happy with having people to stay in her house (covid related). It has come to light that MIL wants BIL and SIL, who moved into their first home about two years ago, to build a granny annexe in their front garden for her, she would sell her current house to pay for it.

As a background MIL is in her early 70s and in generally good health, she had cancer 15+ years ago but is fully in remission. Pre-pandemic she was full-time carer for her own mum who lived next door, but now other family members have taken this on and she isn't involved in her mum's care at all at the moment. MIL has been extremely anxious about covid and I think her MH has been poor. She can be quite a challenging character and has just one close friend who lives locally. She divorced from FIL nearly 25 years ago.

While we were visiting I had a long chat one evening with SIL, who I get on very well with (and she and BIL are fab with our children). SIL is very unhappy about the prospect of this proposed granny flat project, but says that BIL feels that he owes this to his mum and can't say no. I think it is causing some strain in their relationship. DH seems to be sticking his head in the sand about it, and has made non-committal noises when the subject was raised.

My own parents are a similar age and have no expectations of me providing care for them - we have had some roundabout discussions. I realise it's different for MIL as she's on her own, but I wondered how reasonable it is for her to have these expectations?

@Nsky

Care villages and flats can cause issues, tend not to sell well, a bungalow or flat is a far better idea. She needs friends too, shy can’t expect this from her son

I see this said a lot but if she’s there for the rest of her life then it doesn’t need to sell well?

My mum lived into her 80s in her own home (downsized to a one floor flat) and only needed any form of care during her final illness. Not everyone needs or wants sheltered housing.

My oh's grandparents built a chalet on the side of their dd"s house (oh's aunt) grandad died, gran is on her way out and it turns out that because the chalet is on their land it legally belongs to them and they have absolutely no intention of selling their property to give oh's father his share of any inheritance.
The grandparents didn't ring fence the chalet money wise & the aunt says that she deserves the chalet as she's being doing all the care for the last few years.
From your SIL's point of view she could end up being the main carer for your mil, this whole situation needs a serious rethink in my opinion

I think this is a case where you need a crystal ball. If we had built an annexe here and housed MIL it would have worked fine. She was active, happy and had no dementia until she had a stroke in her eighties and died the next day. A good quick death. However if we has housed my mother I would have ended up being a carer for over a decade to someone with complex medical needs, dementia and a demanding personality who is going strong in her nineties.. not such a great outcome.