My mother needs assisted living but my sister wants her to continue to live with her so my mother can pay her mortgage. How can I deal with this?

My mother needs assisted living and has enough income to go into a nice facility. My sister wants my mother (age 84) to continue living with her so that she (my sister) will still get income from my mother to pay my sister's mortgage and my mother will continue to pay for her car. My brother has already blown my father's life insurance so he is not worried about the situation at all. What should I do?