My sister wants to have my parents declared completely mentally incompetent.

[hollysmom]

2. As a quiet non-confrontational person who does hold some resentment

I can recognize my own issue with writing letters to people after I see them. That is a non-confrontational move. The thing I don't understand is why you parents are living with your sister instead of anyone else.

I can also see some solutions to some of the problems - like getting your Mother head phones so she can listen to TV and not bother anyone else.

As the daughter who lived closest to my parents, I had the responsibility of taking care of them since I was in my late 30's and still married. Much like when my Mother-in-law was dying I was the one who visited her twice a week and talked to her on the phone every night for over an hour. Yet when my parents got ill, my husband resented my visiting them even though their house was on my way home from work and the traffic was usually bad, so no big deal dripping by a few times a week to help my Mom take care of my Dad, who was in late stages of Parkinson's. After my mother died, I tried to talk her into moving in with me and she loved my neighbors we would throw parties and invite each others over to the houses and have impromptu dinners in my back yard (with the neighbors on each side when we were all cooking at the same time, just bring your food over and we would share. - the last of the really good neighbors, but she was determined to die in her house.

Here is the thing, - you can't say it is a little money if your patents have 2100 a month after bills - that adds up. One way to control it is to take official responsibility, so money can be a thing, but still, why do your parents live with her instead of one of the other children?
Now, when my mom refused to move in with me and I was working and divorced, I hired a companion to drive her to hair cuts and eat lunch with every day. It worked out well because after the stroke, my mother lost English, but remember Polish and the companion could speak to her in Polish. So, maybe that is the other solution - hire a companion for your parent have to take them where they need to go and not to be stuck in the house.

I hope this helps, it is hard when you don't know the people, my solutions worked for me, maybe they will inspire some different ideas from you.