T.O.
If your husband is the only sibling, then the burden is harder. If he has siblings.... they should all agree on an option to present to the parents. You can't "make" them do anything.
However, you can tell your husband "No" to moving into your home. If he's so gung ho about having them close, let him find a single story or apartment nearby that he can drive to in a few minutes.
I have in-laws just like yours, but fortunately, they live in California and my brother-in-law has taken up their care. We send money when needed and visit.