How do I deal with a narcissistic mother and sister spreading lies about me and trying to take my inheritance?

My wonderful Dad died two years ago and he warned me that I would have trouble with my Mom. She is now almost 94. My sister and I do not agree on anything. I am level headed and have been a teacher for 30 years. She has hopped from job to job and is unstable. She is also bipolar but will not face that. After my Dad passed I spent 6 weeks getting my mother set up in an Independent Living community with the "bribe" that we will keep her house and she can move back if it doesn't work out. I stayed with my mother in the facility for almost a month to make sure she was comfortable (I live a 3 hour plane ride away). My mom is extremely selfish and difficult, but somehow agreed to try it. My sister did not want her to go and she went to the facility and threatened them. (They had to call the police to have her removed). I had things packed up in the car and she kept taking them out. She even threw their wedding picture in the mud. The police were called again., she left for a while but came back. Two months later my mother told my sister to take her out of the facility. My sister gave up her apartment in another state and moved in with my mother. It has not worked from Day 1. She completely took over all the finances (there is A LOT of money involved). My mother has been declining. Under my sisters' tutelage, my mother has fallen several times and even broke her arm. Every other day my sister threatens to leave. She often leaves my mother home alone (and she only can use one arm now). Things have been on a downward spiral. I have tried to help, but it is just not possible. I told my sister to take a vacation and I will come and help. She gave me one day's notice and I came anyway. While I was there she said she is relinquishing her responsibilities. I started interviewing caregivers. My sister came back and sent them away. My mother is very picky about who she will allow to care for her - she has someone 5 days a week during the day whom she loves, but she needs more than that. Now, whenever I call my mother, my sister picks up the phone and hangs it up. She said that my phone calls are making my mother worse. On Friday she was so mad that my mother was talking to me that she left and did not come home that night. My mother was calling me and different neighbors were calling me saying they were worried but did not want to get involved. My sister wants me to have no inheritance. My father left me with power of attorney - my mother (who is scared of my sister) gave it to her instead. I am so sick and really can't help - they are both mentally ill - but there are two issues - #1 - I am riddled with guilt - #2 - I want my inheritance. Help! Thanks in advance.