Meddling sister in law had dad declared incompetent and may be trying to have Mom committed as well. How can I stop her?

My Dad passed away after being put into a nursing home for the rest of his life. He was in a nursing home in the 1940s when he had polio and never wanted to go to one. Mom had a stroke and lives with me. I have her POA but my sister in law has a lot of time to research things on the internet and make calls. Taking care of settling my Dad's estate, caring for my Mom and working a full time job doesn't enable me to find ways to keep her from putting her nose into our affairs.

They live in a different state and I have been limiting communication. My brother never calls just contacts me through Facebook. He wants the title to my Dad's truck which they gave him so he keeps harassing me about that. I am afraid once I give that to him he will start trying to get Mom and her income. He has already said that I don't need to pay her bills because they can't find her. I pay her bills and her caregiver and have a small savings account built up. I am trying to take care of her finances the way she would have.

How can I protect her and myself from their prying? They want to come see her this summer but we don't trust them to be in our home. We are selling her home so they won't be able to stay there, although she doesn't want them in her home. They were here last summer and loaded the truck up with her belongings but did not ask. She does not work, he works 20 hours a week. They live in a housing project and like their life. They are acting as if she is already gone. Any advise???????

I am at my wits end. All of the responsibility is already difficult but manageable. I don't know if I can emotionally and physically handle dealing with their interference.