How can I stop my sister from overiding my decisions when I have POA and PD?

My mother is 101, and has Borderline Personality Disorder and narcissism, both for her life time, and, in recent years, increasing paranoia. She is well situated in an ALF. This summer, after some troublesome events, her lawyer activated the EPA (enduring Power of Attorney) and PD (Personal Directive) on which I am named as mother's agent. Mother's paranoia has focussed on the ALF staff thinking that they steal things from her, over charge her etc. and now it apparently has extended to her bank. Everything that mother has said was stolen has been found by the ALF staff in her unit. I live 5 hrs. drive away and am 76 myself. My sister, who is a year older than me has never been helpful. I have always been the one asked to do things - right from our 20s.I moved mother from her apartment 4 years ago to the ALF of her choice. That lasted 6 months, when there was in incident with a staff member. They did not handle it well, neither did mother (no surprise) so mother was moved to another ALF of her choice - the one she is in now. The staff in this ALF have been nothing but professional, kind, concerned for mother's best interests, and although she has caused them some problems, they are willing to work with the situation. She has been there for over 3 years now. I am very impressed with them. Her case manager recommends that mother stay where she is, as the alternatives are not as good. It is reasonably expensive, but mother's financial manager has assured me that mother has enough money to last another 8 years clearly, and more. Due to her paranoia, mother wants to move to another ALF, which, incidentally is cheaper, though that is not mother's reason for wanting to move. She calls the staff where she is "thieves". My sis has similar mental problems to my mother and always has. A psychiatrist who saw all of us years ago pronounced me normal. my mother mentally ill, and then said he was more concerned about my sister. He had gone to see them after seeing me and they threw him out after 10 minutes,
After the fiasco of last summer, mother had settled down and was content to stay where she was. As long as she takes the antipsychotic drugs, her life is manageable. Then her behaviour started changing for the worse after my sis, (who lives in Scotland) planned a trip over. I went down to visit mother end of October and she refused to see me or to answer my phone calls. The purpose of my trip was to obtain info regarding her insurances, pension people etc and get her business mail redirected to me. She is still very bright, according to the drs. borderline competent. I work to get her agreement in these things before I do them and I had her agreement to take over her business mail. My sis planned a trip over here for the end of November without consulting me regarding dates then she and mother were mad at me for not being available. At the last minute, one of my plans fell through so I called mother that I was available and she refused to see me again. That was followed by an abusive phone call from my sister who claims that I have not been doing my job as EPA etc. I should mention that when I first moved mother my sister was visiting and literally did not lift a finger to help with the move, but sat and watched us work, and went home with the most expensive piece of mother's collectibles. The next day I had 9 phone calls, which I did not answer - each voice mail crazier than the next and begging me to come down and move furniture??? I contacted the ALF staff and mother's case manager who told me that my sister has been judgemental, demanding, and critical with them. She is trying to get mother into a cheaper ALF for which mother does not qualify, as it is for people with more disabilities than mother has, and has demanded that the case manager make an assessment that will get mother in there. It is not going to happen. All this after I told my sister that my decision as mother's agent was to keep her where she is. Where ever mother is she will bring her problems with her. In my mind keeping her where she is where the staff are good and working to get her back on meds is the only answer. Sis recognises that mother will be unhappy where ever she is. Mother’s case manager has called the mental health nurse to evaluate mother and possibly admit her to a psych ward for stabilization and treatment. The staff all see that mother has regressed since my sister’s visit. Sis has returned home and will be back probably within the month to move mother. Although she cannot move her to the place she is trying to, there are other places she could move mother. I have told the ALF not to accept any termination of lease there without my approval. That will not stop my sis from moving mother if she can. I suspect that I need to ask mother’s lawyer to send my sister notice that she does not have the authority to move mother. I am Canadian and we do not resort to using lawyers quickly, and am looking for any ideas of how to deal wi