How to handle a very difficult, narcissist, older sister who does reckless things? My elderly mother is concerned.

My 89-year-old mother has advanced dementia. Long story short she has caregivers which I am very thankful for, but my sister is very harsh with them and always finds something complain about. The current caregiver cooks clean and takes care of our mother 10 hours a day. And she attacked so badly that the caregiver called in sick but admitted it was due to her actions. She goes out of her way to create drama for no reason. We recently were on a trip, which she was jealous of, and the day I got back announced she was leaving for 10 days. I live and work in another state and need advance warning to plan in case I need to be there. She just threw me under the bus. She resents that she’s a block from our mom, but I’d do anything to be able to pop in but instead rely on FaceTime and visits when I can. We wanted to sell my mom’s apt which we co own and move her near us, but my sister wouldn’t agree and now complains when it’s the caregivers who kill themselves for my mom. One of the caregivers stood up to her and she did everything she could to drive her off although she was so loyal to my mom. It never ends. But I heard that there is no reasoning with a narcissist.