My fiancée's mother is in a dementia care home. He lives in her house and would like me to move in. How can he get enough power over siblings?

This will depend on whose name the home is in, who as Power of Attorney for his mother (if anyone), how her will reads if she has one, and other things. If you want this to work, you should work with an estate attorney or elder law attorney.
Carol

Buy your own home.