I would support the two younger sons who are trying to move out, see if there is room for them in your/their older siblings' homes maybe. It does not sound like your father-in-law is very stable or loving in his behavior choices after his wife's death but I don't think anyone else can change that and, legally, he can do whatever he likes with selling the house, etc.

The boys are almost grown, they will have to deal with their father's behavior at some point on their own; between him and them individually. I would just see if you can get all the people who are appalled at his bad behavior on the same page helping the two younger boys get on with their lives now so they can mourn their mother, finish school, get into their adult lives.