My Grandmother is in her 80's with possible dementia, she has my aunt (50) with mental disability. How do I protect them?

The sad reality is that your Aunt will face separation from her mother eventually. If Grandma has resisted change for so long it may be impossible to get trough to her now that she is slipping into dementia but I think it is worth a try. Can you sit down with her in a loving way to try to discuss their future? I'm sure that this has been a worry to her for many years so she may be thankful to have someone help her make a plan. You will need to start to get all the legal ducks in a row, POA for your grandmother and guardianship for your Aunt (if that is what you are willing to do). I personally feel it would be unfair to keep her with Gma if that would mean a move to a senior focused assisted living or nursing home, she should have the opportunity to enjoy the company of her peers. However, when a relative of ours needed care the family was able to move both of them to a facility that offers multiple levels of care.