How to deal with mother in-law in 80’s who cares for son with Muscular Dystrophy and can't really take care of both?

My brother in-law has had Muscular Dystrophy since a child. My MIL has always chosen to care for him; He is now in his 4o's and her now in her 80's with extreme Rheumatoid Arthritis. BIL requires around the clock care now because of tracheostomy & Ventilator. My husband takes care of him every night from 10pm- till 10am and that is if she doesn't have appointments to go to or errands she needs to run (because she doesn't drive).

Once my husband leaves, she is left to care for my BIL during the day till my husband returns. She is able to meet simple needs but unable to help him with major care issues like changing his cannula or if he needs to have a bowel movement because she would have to place him in a hoist. She can pull a leg if needed to adjust, feed him through the peg tube that he now requires to be feed through or have him pee in a bottle.
But if anything major arises one of us has to be on call at all times. So needless to say we have not had much together time since my FIL passed over 10 years. We are now getting concerned with the Physical Wellbeing of my BIL as well as hers. He has no ability to call for help if needed and she will not allow anyone to come in during the day and threatens she would rather they both die than have him go to someone else. I know that my BIL is just as dependent on her as she is him. In his mind for she has told him often enough, she has sacrificed everything for his care. So he will not say anything for fear of offending her or making her angry. I know if I call someone in to investigate she will turn on the charm and then they will close the case and then I have other issues to contend with.
The conflict between us is there all the time! I know she is depressed and will not allow us to get her any help. She forgets where she places things or to pay a bill or two but nothing dramatic in that department. I even do that and not in my eighty's.
We are just a loss in what to do and how to be able to protect them both. But my husband is the only child to help there are no other siblings. His mother never forgets to remind him of his loyalty to his family. Mind you we’ve only been married since 2000 and his father died a year later. She’s been sucking the life out of him since. We have suggested they come to live with us so there would be more people to help as well as to share the load and again she would rather die. She wants her independence and in turn holding us hostage. I just want my husband back! I find myself resenting him and yet the flip side of the coin admires him for it. Any suggestions would be so greatly appreciated.