This has been a growing problem for years and even though Dad has been practically bed-ridden for more than two years and Mom is worn out, they continue trying to keep a failing business afloat (going into debt) in order to have another building for housing the cats. Although they would say otherwise, the cats are their life. They have pretty much lost their life savings, put the health of the cats before their own health, and may eventually lose their home. A few years ago it seemed they realized that something would have to give, but now it seems like there is nothing any other family members can do except wait for something catastrophic to happen.
Perhaps in your area there is an agency that will take on the cats? You didn’t indicate that they were endangered in any way. Regardless they might help relocate the cats. It seems like it would be beneficial to at least know the options before that catastrophe occurs in order to take the cats quickly.
I know this must be a very stresssful problem for all concerned. Good luck and let us know how things go.