Make the things she does like (so she'll eat them, later)and freeze those. If you're not sure how to freeze certain foods so they'll still be tasty later when thawed, there's a wealth of related information on Google search.
It's all about learning to trust the right person.
You might try asking your mom to try 'outside help' for just one- or two weeks, and see if the 'right match' assistant shows up and is able to change her mind.
Let the other family members try...and see what happens.
You also might try consulting with an experienced Social Worker.
One important one, to me, would be "What kinds of things are you going to do with my mom (or dad)during the day, to help keep them mentally and physically active?" (After light housekeeping, etc.)
(Probably because they never learned the meaning of the concept "being ethical.")
I would definitely hire a lawyer.