MIL is quite bad, her husband (FIL) died of cancer 18 months ago and at that point noone could pretend she didn't need help (even in DP's family of emotional illterates who all avoid talking about anything remotely awkward). She was assessed as needing loads of care, she has to pay the top rate any old person is charged for it as she has lots of savings, but still lots more of the care comes free, paid for by the taxpayer.
DP is in charge of her finances and yes he does worry. He worries that all the money will go on the many hours of care, but he doesn't like to think he's worrying about his own possible inheritance, so he frames it as worrying about whether his mother will run out of money to pay for her care (that's unlikely, impossible even, she has loads of savings and a house which would be worth a lot if it doesn't totally rot around her due to not being looked after at the moment, but of course DP isn't being totally rational about this, he worries about the money because he finds that easier than worrying about his mother in other ways).
His sister gets to do the worrying/arranging about the carers, this is a problem as they come from agencies and seem to skive when not checked up on. But, poor carers, they are supposed to come for 3 hours every afternoon and MIL can be rude and ungrateful to them. That's the alzheimers, she's not a rude person naturally.
and his brother lives closest and gets to do the popping in and doing emergencies.
it's a mess. and it takes a lot of energy and time and money from all 3 siblings and the carers etc. but what else can you do? It does seem she doesn't want to be moved to a home, so maybe we/DP and siblings have to just go with it.