I am noticing that you live in PA (the state where I was born but not where I am licensed) so I can only give you a California answer, but hopefully some answer is better than no answer.
Here, in California, this conduct, while disagreeaable and unpleasant, would not rise to the level of qualifying her to be a Conservatee, with someone else in charge of her Person and Estate. She has a right here to make poor decisions, which means that she suffers the consequences of her decisions. There, however, the whole family is suffering the consequences, which doesn't seem fair.
Has she seen a doctor? Can you get a physician or pyschologist to come to the house to see her, to make a diagnosis? This sounds a bit like Mental Illness, but I am not qualified to analyze that. You may want to contact the local chapter of NAMI/National Alliance for the Mentally Ill, to get access to local resources. Sometimes these senior mood swings (and disappointment with losses) can be resolved with a full medical work-up, a nutritional review, and medication for missing chemicals in the body for the parts that may no longer be working as well as the rest of us does earlier in life.
I'm suggesting this last tactic b/c you mention that she keeps 'coming up with medical conditions' so possibly this is the door to getting her to change and move on. But the suggestions have to come from the medical providers, not you (which they can do if you give them the informaiton). Let them guide her away from you.
And if she has to go into the hospital for tests, you can change the locks on the doors of your home, and tell the discharge planner than she is unable to return to your home ~ and why. Then when she is in a different place, you each can work out more clearly what you want your future relationship to be, and become.