I think you really need to see a solicitor on this one.
I returned home to live with my father after we were both widowed within a year of each other.
I had been left my mothers share of the house so the house was owned by both of us.
We asked a solicitor what would happen if my father had to go into a nursing home and he said as I would be an owner occupier with the right to live there and without my consent to sell my fathers share of the property would be worth nothing on the open market and as long as I kept the property until after he passes away it could not be claimed by the council.
In your case it appears your mother in law wants to sell a house in her sole name and buy a bungalow in joint names.
I think Social Services may argue deprivation of assetts has happened. On the other hand if she has no reason to think she will need care as of now she does have a right to gift some of her property now so that is why I am saying see a solicitor.
Martin