Do we have any legal rights to help my father in law from being taken advantage of by his sister?

My Father-in-law is 77 and health is declining dramatically over the past year. We live 2 hours away from him and want to move him to our town into an assistant living so he can receive the care the needs.

Here is a run down of what we are having a hard time with, and don't know what to do. All this has been brought to light over the past 2 months. My husbands family is VERY toxic. We only speak to my FIL and a niece who is only 17 and living with family members from her father's side out of state.

Over the summer my father-in-law's sister past away. She had a 56 year old mentally disabled son (I will refer to as J). My FIL's other sister, her girlfriend, and his daughter drove across country to get the son. They attempted to empty the dead sister's accounts with no luck, but were able to clean the house out of anything worth value. They took the son (J) and left her in the morgue where they never claimed her body. When they came home he lived with the sister for a couple weeks, and when they discovered they could not receive social security benefits for him, they dropped him off at my FIL's home and told him they can afford him. My FIL cannot care for the disabled man. On top of this we have learned that the sister is trying to get my FIL to change his will to leave everything to her. We received a phone call from our niece who was visiting my FIL, that the aunt picked him up and took him to an attorneys office. We are unaware if he did end up changing his will. On top of this, I just got access to my FIL's medical records and they have my FIL's sister listed as his wife. We are just starting to realize what has been going on, and really don't know what we will find next. My FIL is having a hard time understanding what is happening lately and just does what anyone tells him "needs to be done".

We have been trying to get the 3 responsible for bring 'J' home with them, to take him, but they refuse. "J" has no income, cannot work, and really needs to be in the hands of someone that has knowledge of how to care for a mentally challenged adult. He has started several fires (unintentionally) in my FIL's home. Has gotten out at night, and has almost die of a skin infection do to improper care. We cannot care for "J" and we need my FIL to move close to us so we can get him the help he is needed. But by doing this we need to sell his home and use what little equity is left to get him the medical care he needs and we will pay for the rest.

So here's what we are wondering, if someone has gone through this before.

Is this something we can report to an agency? And if so who? Can they even do anything?
Is there anything that can be done to stop the sister from having him list herself on accounts or changing will's?
How do we get "J" out of the house, but also cared for?

A couple side notes;
We need to sell his home in order to pay for the assisted living. We do not financially have the means to pay for it other wise. We are coving medical bills for him now, but can no longer do it.
My husband has two sisters. Both do not speak to their father. One stole all his retirement and saving about 20 years ago. The other is toxic and he cut her off.
We have no access to "J" medical. We offered to help get him help and the aunt tells us to stay out of it. My FIL saids he doesn't know what's going on.
The aunt just popped up about 3 years ago. I had never met her and my husband only meet her once before. She moved to his town stating that she was going to take care of him. Now we are seeing what that meant.
I know we where very naive to think she was going to help. I've never been though this, nor has anyone on my side of the family.