Dear Rumpled...I'm sorry to read about your pain and MIL sufferings, all of you are suffering.
All I can offer is a "cybr" hug....I know thats not much help....but

As I was reading about your worries...I hear your deep compassion and frustration, concern, love and caring for all involved.
Your family is blessed beyond measure to have one such as you!

I was reflecting on what my councilor would say/advise....to let some things "go" when I had such concerns!
That some "Things" weren't really that important...such as the time or what they ate....clothing, hygene etc.
Was NOT my values or expectations.
I had a hard time with such thinking...especially knowing what a mess their lives were....
one of them, not eating on time....or their hygene being something we would never want for ourselves.
But...YOU must take care of YOU!...need to take some time for self care and let a few things go!
I could do nothing to help their situation...and just spun my wheels with worrying, being upset over what "was and wasn't" going on under their roof.
(this involved small children was heartbreaking to me)

There's got to be a balance in here somewhere...and trade off for letting "something" go...will be in giving to yourself...maybe with "time" or any way that would bring you peace or happyness..
I don't mean to be dimsissive or unfeeling....I have been in similar situation as yours...only different faces/places,,,but same ideeas.
Blessings, cheryl