My sister won't let my mother move into a retirement home. She is tormenting my mother and me for putting down a deposit on a place.

My wonderful father passed away in April. My mother, who is 93, relied on him for everything. He took care of the finances and even did the laundry and made the bed. My sister stayed for 4 months and "took care of" my mother. She did do a great job with all of the clerical work involved when somebody passes away. However, she did not do a good job taking care of my mother. My mother is extremely difficult and was not nice to me at all growing up (and still is not). Long story short, I am a teacher and came down to Florida from NYC for three weeks to stay with my mother. I did not want to but felt it was the right thing to do (I was ready to give up my job and come down for my father because he was so wonderful, but she truly hates me). My sister has no job so she was able to stay, and now she went back to NYC for these 3 weeks. All along we have been looking at Independent Living Places as an option for my mother. One of the ones we saw when I came was perfect for her in many ways. When my sister was here my mother called me crying every day about how mean my sister was and how she wanted her to leave. I believe my sister is bipolar though she is undiagnosed. Her current neighbors have been coming up to me separately telling me tales of how my sister is not looking out for my mother in many ways (making her walk places etc.). My mother put the deposit down on this place and my sister went ballistic. She called the place and harassed them. She leaves harassing messages on the answering machine - she told my mother that she has a lawyer and that my mother will be dead within a month if she moves to this place. She does not explain herself, nor does she offer any other solution for my mother. My mother is afraid of her and does not tell her how she really feels. My sister said she is coming down Monday. She won't stay how long she is staying. I want to leave so badly, but I planned to help my mother pack and move and do all of the work involved. I cannot stay here with my sister here. My mother plans on taking my sister to see the place to get her approval, but I guarantee you my sister will make trouble when she goes there. My sister will not talk with a mediator. My mother is afraid to move without my sister's approval. I am sick all of the time dealing with my nasty/ crazy mother and sister. What can I do?