Then my fiance is ALSO being expected to take up that responsibility because all her siblings went and had 2-4 kids and “can’t afford” or “won’t have time” to help take care of her mother who is already aging.
Your fiance’s siblings are stupid for turning down the free live-in childcare.
An elderly person verging on disabled and one day probably will be, also with what looks like developing early onset dementia probably doesn’t make for good childcare
Someone with dimensia is great for occupying a toddler/young child though! When I visited some family my at the time 3 year old and my grandmother and her sister in law spent 2 hours setting the table because they kept having to re-count how many people they have, how many places they have set, and kept confusing each other and having to get it all straight again. They all seemed to enjoy the time together and it was fun to watch it all unfold