My sister is revising all my parents plans - I just can't deal with it

I moved both parents to an ALF three months ago. They both have gone downhill rather quickly. Mom has stage 5-6 dementia. Dad has COPD and was recently diagnosed with CHF. Both are double incontinent. Dad is showing signs of dementia too. Their living conditions are challenging.

Ten years ago, they did their Wills, DPOA was in place (naming both my sister and I as secondary contacts after their spouse), Patient Advocate forms, and they paid in advance for their funeral service and cemetery plot. Dad gave me all these forms and for 10 years I have held them ready to refer to when needed. My sister never even asked, nor was she involved in any prior conversation with my parents about their plans. She stated recently she never even knew she was on their POA forms. She showed zero interest.

The ALF called two weeks ago informing dad's CHF was worse and now he is in early renal failure. They suggested he go to ER for in-patient treatment again but warned it could impact his kidneys. We informed dad about his condition, and he stated he did not want to go to ER. He said "no doctors" poking around. He also told us we could drag him out of the ALF when he dies. I supported dad's decision. My sister said she wanted him to go to ER, but would also support his decision. I said I would make arrangements for Hospice care for dad and up until yesterday, she told me she agreed. So I started to set up an eval with Hospice.

Now I see my sister is going in her own direction. I question why? She never even visited our parents that often and now she is all over all details of their life. Is she on a power trip, looking for money, eager to exploit? Is she eager to take away something from me to feel superior?

Yesterday, she started texting me repeatedly asking for all the original paperwork I was holding. I refused to give it to her. She showed up at my house this morning and texted she was in my driveway and to give it to her. I said No. She stated they needed the POA revised since both are doing so poorly and cannot make their own decisions. I refused to agree. She informed me she brought an attorney to my parents to sign "all new paperwork". I have a job. She does not. I cannot take time to deal with her whims at random. I went to visit parents this evening and ran into my sister on her way out. She haughtily stated since I did not give her the original paperwork, it cost them more money to be revised. She was accusatory and aggressive. I was livid, sad, and appalled.

Before she walked out, my sister casually said she is going to 'take dad to ER tomorrow'. This is not what we agreed to 12 hours ago. All the plans I started for Hospice are irrelevant if she takes dad to ER. I asked if dad told her he wanted to go. She said she had not discussed it with him. She said she just decided he is going. She was there with her teenage daughter and I did not want to get into an argument so I said nothing. My sister then questioned the cemetery they had chosen to be buried in and said when dad dies she would just have the burial somewhere else close by instead. I was again appalled. I said no, but she just shrugged and walked off. She acts entitled and exploitive. She has no boundaries. She has no empathy or regard for our parents - or me.

My first question is - could there be any honorable intentions behind her acts to revise all of mom and dad's plans?

My second question is - can I stop her? I do not think I can as long as her name is on their POA. I talked to a lawyer a few months ago, and he told me my options were to 1) have both parents remove sister's name from their documents (they would never agree to that), or 2) I get two doctors to claim both incompetent (dad still is aware enough where I do not think that would work).

In addition, my sister is also the Executor and their Trustee Successor. My parents have always adored her as their 'first born'. She has always been their favorite. They never see her manipulations. I have tried in my life to tell my parents what I see in her, but they have never agreed with me. In fact, they always just got mad at me. I have learned to let so much just go because I am trying to save people who do not see a problem. Even I try to 'save them', I am not so sure there is any actual gain anymore.

Do I have to just let this go? Thanks.