Dealing with impossible family member! Need serious advice!

This is going to be a long post so please forgive me but I do not know where to turn to and really could use some advise on my situation. My husband and I moved to Florida 2 years ago. We moved in with his elderly uncle and aunt just until we found a place, jobs, etc. Well the aunt who was already very sick with cancer got worse. The uncle was still working at the time. We were asked to stay to "help" out. So, we stayed. We were not asked to pay anything because we were providing all of the help in the home. They even let go of their weekly housekeeper who cooked, cleaned, did laundry, etc. Fast foward one year, the aunt passed away and the uncle has been diagnosed with dementia and it's pretty bad. Now we are having to deal with his son who in my mind is nothing short of walking evil. He has come in and taken over everything. He has taken his father's bank card and gives him $100-$200 monthly for expenses. He pays the utilities in the house, except for the phone, internet and cable as my husband and I pay that. We buy our own food and essentials. We also do all caregiving for my husbands uncle. We cook, clean, make sure he gets his meds, we clean his room, bathroom, the pool, we do his laundry, make sure he showers, etc. You get the point. We also work full time jobs but work different shifts so one of us is always here with the uncle because he cannot be left alone. Now, the son wants us to pay the full electric which he says is $350 a month but never shows us a bill. I refused. I said I will not pay anything without seeing the bill. He has since cut the AC off and removed the batteries from the thermostat. Sure, we could put some back but it's the actions that bother me. He tells us he wants us to pay $500 monthly in rent and sign a lease but the uncle says no. He also says if we leave, he will die because his son will put him in a home or something. We do not get paid to be caregivers, we do it so we make sure the uncle is cared for. His son does not help him other than calling daily, he runs by on his lunch break during the week, but hardly comes around on weekends (he's 40, single and loves to party). He has taken full control of his father's finances, which I do not have a problem with except he does not shop for his father, we help with that. He took his bank card, all bank statements are sent to HIS house after I checked one once and saw AT&T, Directv being paid from uncle's account. Uncle has neither but his son does! He takes him to the bank and has his dad sign a loan, he said for back taxes his dad owns. I really do not know details of everything but it doesn't seem right to me. I am at a point, I want to move out and leave this mess with his son to take care of but the family begs us not to because in their words "his son will kill him". They say he is just waiting for his father to die because he will get all of the homes, business in New York, etc. We do not want anything from anyone, just to be able to make sure uncle is getting the right care and not being taken advantage of. But I do not know how to go about finding out IF his son is indeed misusing his father's finances. He doesn't do anything else for him, only deals with the finances and fills his pillbox weekly. He might bring him some food occasionally and will bring $100 when his dad (uncle) says he needs money. But he tells him he needs to make this money last and he needs to learn to do things on his own! This infuriates me because the uncle has dementia!!! He cannot help his condition. The son does not like me at all because I have expressed myself to him and what I think he is doing and he literally cusses me and says WE are using his father, looking to stay and not pay for anything. That is NOT the case! We tried leaving several times only to be begged not to go. I do not know how much longer I can handle this. I am almost certain his son is misusing his money but due to uncles dementia, I do not know what can be done! Is there any rights for us being his caregivers and living here in his home or can the son kick us out? I really dont mind leaving, but I dont want any surprises. Does anyone have ANY advise or suggestions??? If you made it this far, thank you for taking the time to read my story!!