Unfortunately I was in a similar situation with my mother. She was in a memory care facility in NY State for a seven month stay - undergoing medication management (trial and error to find the right "combo" so she could live in a family setting without harming herself or others.) She had early to mid-stage Alzheimer's at the time.
I had been given durable power of attorney for finances, health and what-all for my mother. I am the oldest, live in California, and had long been estranged from my parents, but BOTH my parents had asked me to fulfill this role in their lives, and when my father died suddenly, my mother held to the same decision for herself. This ENRAGED my sister, who had been emotionally dependent on my mother her whole life, as well as financially dependent (unfortunately, NOT out of honest need, but out of greed, mismanagement and self-indulgence.) My sister lives in Florida.
My mother, already confused and easily swayed, was also dealing with shock and grief over my father's accidental death and believed anything anyone told her. To the extent that I was responsible for her, I needed to earn and keep her TRUST, so she would feel safe and secure.
My sister did EVERYTHING she could think of to undermine my position of responsibility with my mother. I did NOT bar her from visiting, calling, writing etc., but DID bar her from taking my mother out of the facility. Her contacts with my mother made my mom agitated, angry at me, demanding, and paranoid. My sister kept pushing my mom to request a competency test (which she would NOT have passed at the time,) something I also wanted to do for her, but not until she was stronger, properly medicated and healthy etc. ( When I finally did take my mother in for testing, she actually passed...)
My sister concocted stories for my mother, telling her "you don't belong in there" and that she was hiring a lawyer to "get her out of there" etc. Of course, my sister had no money, so there was no lawyer, but that did not prevent her from sending DAILY REPORTS of the case "progress" and all MY efforts to quash it! Since much of her contact was from a distance it came in the form of cards and letters, which my mother was able to read and re-read, again and again. It was AWFUL. At one point, my sister even called the facility, pretended to be ME, and tried to get my mother transferred to one of their sister-facilities in Florida, where my sister lives. I discovered this plan by a "fluke," after which I needed to take SERIOUS action against my sister - with the help of an attorney.
I love my sister, but she made an extremely difficult job even harder. In addition, her "antics" cost my mother over $20,000 in legal fees, as I needed to hire an elder-law attorney to assist me with handling both my sister AND the memory care facility to make sure my mother was adequately protected.
Was this about money? From my sister's perspective, probably. But from my perspective it was about interfering with MY MOTHER'S AUTHORITY, which by my mother's own choice, she had vested in ME. It was also about my mother's mental health, which was already compromised by her grief and the Alzheimer's Disease.
Hope this helps someone.