It's a while since you started this thread, and you may have already sorted out what to do. In case things are still up in the air, I would make one suggestion. Do not get talked into having your MIL move in with you, if you believe this would be overly burdensome on you . . . which I'm quite sure it would be.

It does sound like early signs of dementia. Often that can only be recognized in retrospect, after it has progressed a lot further. Even a doctor may often not be able to diagnose early dementia. (I know that first hand.) Regardless of where she is cognitively, she clearly is decompensating emotionally, which can amount to the same thing. She obviously is becoming unable to deal with being alone. It may be that she can't help that. Perhaps excessive dependency is a part of her natural makeup. In any case, that's likely to snowball into a bigger and bigger problem. At some point, getting her placed in a supervised setting may be for the best . . . though it can be expensive. Sounds like she is open to that.

It's really up to your husband and his siblings to figure this out. Your main role, right now, is to remove an option you're not comfortable with - having her live with you. That's your right.