My FIL is 86, active and healthy. He officiated high school football thru age 85. COVID forced him to retire. He still drives but we are talking to him about giving it up soon. He will live well into his 90s. Longevity is in the family genes. He was recently scammed out of a good portion of his money and it became apparent that he needs to be watched over more closely. We have been discussing this move with him for the last two years since my MIL passed so it is not sudden and he is willing. He will pay us a monthly room and board. We have spruced up his room with new carpet and drapes. We already have an extra large walk-in shower but we will need to expand our half bath into a second full bath with an accessible shower. I don’t think we will need any other accommodations to our ranch style home but he will contribute to these renovations as we are retired too. My concern is that I do not trust my SIL or BIL and want to be sure that hubby and I are protected from accusations of abuse or financial impropriety that could ruin us.
Your father in law will not be getting better or remaining static and you need to understand that things now will decline, not move up. All your contracts signed sealed and delivered work ONLY SO LONG as your FIL chooses to have you serve as his POA. Or until he can no longer, in the opinion of several doctors, act in his own competence.
You need to understand that SIL and BIL have no say, no rights to know anything, and have only the right to ask wellness checks from APS. IF/when this happens bring out all your documents and welcome them into your home. Then that will be over with quickly.
You need on your own to discuss boundaries. What private time do you and husband require together, unless there is an emergency. And etc. Basically anything you can think of to add to the list add it.
Basically make sure everything is spelled out, keep meticulous records and don't worry about the rest of the family - as long as you you have good records and do your best by FIL you should be fine.
At some point in time he may have to go to nursing home - unless he has enough money to cover 24 hr care. You need to be sure all your t's are crossed to avoid any financial problems or assistance with Medicaid if his money runs out.
What help are they offering toward their father's living situation and care taking needs if they develop? You have not indicated that they are involved.
Keep careful records of how your father:s money is used, whether for "rent" or needed modifications for his own safety. If you have careful documentation of care and expenses, any frivolous accusations his other children may make will prove to be unfounded.