Sorry you're going through this. My family always had plans in place, and the older generation sat down with the younger generation and spelled out what was where, what policies existed, and so on. Sounds like the 2 brothers in your family want the sister to handle everything, but she's not able to OR there's just too much to handle. In my family, we've got someone from out of state trying to rush in on the white horse to save the day, unaware or unconcerned about what's already being done, not to mention unclear on what kind of condition the older person is in.
It's so very hard to view one's parents as compromised - and often the elderly person wants to stay in their own home or just has no idea how to proceed. They often don't want to "burden" their own children and still see themselves as the caretaker rather than the person in need of care.
We've be working a lot with rehab facilities and elder care, and we've learned that the older person often cannot sense their own failings, has no idea that they don't have the balance or judgment they once had, etc.
I'd suggest that your husband contact the elder care agency in your GMIL's town or state, and find out what services are available. Often getting an objective case manager in there to assess things is a huge help. You might also start with GMIL's primary care physician to see whether there are any orders in place, such as health care proxy or even do-not-resuscitate (DNR) or do-not-intubate (DNI). Usually doctors are checking with their patients on these things. These professionals can help connect your husband or his aunt to services in the area. Your husband's father and uncle either need to drive/get on a plane and sit in the room with their sister and their mother and a professional, or they need to get the hell out of the way.
You can also find out about respite services for your husband's aunt - if she's got all the work, she's got all the stress. I'm going through that myself, and that's in a family where the older person made all the arrangements, moved into a retirement building with lots of levels of services available from assisted living to rehab/nursing home to temporary care to personal care attendants. She's fallen, had surgery, rehab, and 24 hour aides, and she's feisty and difficult, denying she needs anything. Meanwhile she leaves the stove on and does a variety of other things, saying she's not going to fall. Never mind that she's bruised and has broken bones from all the falls she's supposedly not having! And she's good and mad at the family members who are "butting in" to help. So even when plans are in place, they don't always function well.
Good luck - your husband has my sympathies!