Hello, Tom. You are such a loving son to want to help your mother to the best living circumstances in her final years. Kudos to you.
Like the others, I agree that moving her is less optimal than leaving her in her own home. If the home is clean, dry, warm, and not prone to additional bug and rodent residents, then this seems like a good place for your mother to be in, at least for now. Given that this is a progressive disease and she might not always be ambulatory, it would be difficult to determine the best course for any situation that might eventually come your way. (For instance, can the space handle a wheelchair or hoyer lift, if she reaches the final stages?)
Your plan of having a family meeting is such a good idea. I hope this meeting will take place outside of your mother's hearing, just in case it should come about that there are arguments about her and her financial condition or her needs.
I think that if you wanted to seek a rich resource for plans for the future, dealing with differences of opinion among siblings, and so forth, you might want to consult the book called The 36 Hour Day. It is just made for people like you (and people like me).
My mother is in stage 5, also. We moved her to my home about four years ago, when the diagnosis was first reached. She had actually be living with us on and off for the previous two years. With the diagnosis, the family had to act quickly, since she could no longer live in her home on her own. My one sister who lived in that area could not bring herself to move in and live with her and help her. So we concluded it would be best to sell the home and move her in with my husband, me, and our elderly rescued dogs. It has worked out for her.
I mention this mostly because it's not the end of the world for her if it should happen that you have to move her. It is possible that if you and your sister could remain with her, she would do quite well with the change of scene. The book I mentioned offers suggestions about how to help with adjustments after a move.
Finally, I wanted to suggest that, if you have not done so already, you and your family will want to create a power of attorney so that someone can handle the estate matters/investments, etc., in future. And you'd do well to have an advance medical directive, in case your mother eventually needs complete medical care. My mother has rejected what we all call "heroic measures," including feeding tubes. This is something you and your family will want to think about as your mother's situation continues to deteriorate. It is something that, ideally, the siblings all be informed about and agree on - but it is your mother's decision, not anyone else's, about whether she would like life-sustaining measures to be used.
Your letter moved me when I read it, and though I don't come to the site often (I have a full-time job while hiring help for but remaining my mother's primary caregiver), I came back this evening to send this message out. You are a loving son doing what you can to help your mother.
All good wishes to you at this difficult juncture in your life,