How do I put together a family forum to discuss the care of our elderly mother suffering from dementia??

Sometimes having a professional such as a clergy person, a doctor, a geriatric care manager or a social worker to give the meeting some grounding, and take out family dynamics to some degree, is helpful. Third parties are very useful here.
A little more extreme, but possible, is a family mediator. Whatever route you go, a family meeting to discuss this issue is wise. If the daughter who wants to move is reluctant to be in on the planning, she may feel guilty for wanting to move. Try to keep attitudes in check and stress that the whole idea is about MOM, not anyone else. Again, I'd suggest a third party to help, since that makes it less "family" and more official, so people will be on their best behavior (hopefully).
Good luck,
Carol

Definitely use a neutral party to facilitate the discussion --- I can only imagine a fracas happening with a family member attempting to chair a meeting. It's not fair to that family member to put them in that position either. I arranged a Geriatric Case Manager from the Visiting Nurse Association and it was a successful beginning for our family communication. I can't say enough how important it is to have a professional support system like the VNA.