@Waterwahine ,
Wow, your MIL really is treating you like 'the other woman'. Egad, she's awfully emotionally dependent upon your husband.
As for her needing care, she's probably pretty close in age to me (I'm 55). I don't need care. Of course her needs may be different but the bottom line is that the 50s aren't old unless you've got some sort of a major illness. Since you didn't mention it, I am operating under the assumption that she's not a cancer patient, didn't have a heart attack, isn't on oxygen, doesn't have diabetes or HIV, and is in decent health but not necessarily a marathoner.
Her parents, in their 80s, are a different story. That is a decade when a lot of people get impaired in one way or the other.
But she's most likely not.
I bet dollars to doughnuts the idea of moving you into a closer house was intended as a means to get you to shoulder care responsibilities for her folks and, eventually, for her.
You and your husband don't have to do any of this.
Nowhere in the child-parent 'contract' is it written in stone that the offspring must always always always care for the parents and/or grandparents. This doesn't mean you get to have a heart of stone, but at the same time, you and your husband do not need to be responsible.
You can hire help. You can finance part of the care. You can be nearby and visit. But you don't have to change bedpans, etc. no matter what anyone says.
Your husband sounds like a truly nice person who his mother has glommed onto. I take it she has no friends and no social circle? No dates, etc.? Because if she had pals, she could travel with them, etc.
I am guessing (I am not a doctor) that she is depressed at least a bit, and certainly isolated in her life. The fact that that may very well be her own fault is immaterial right now.
Maybe the thing to do is get her in to see her primary care physician. At least rule out organic issues and then, when she (probably) gets a clean bill of health, start pushing her to some activities with pals of her own age. Me? I would not be a terribly good traveling companion for someone in their 20s or 30s and the reverse would be true. If she were to spend time with her peers more, she would lean on your husband a lot less.