Thanks for your thoughts everyone, we do appreciate them.
Outnfree - Grandpa was asked what he wanted to do and he said that his first wish was to live at home with carers coming in to help him and failing that he would like to live with us.
They said NO!
He came to stay with us for a couple of nights and his son threatened to have him sectioned under the mental health act if he didn't return to the care home. My father in law did not know that with our phone every one in the room could hear what he was saying to Grandpa and he said awful things about us and accussed my husband of violence in forcing him to stay at our house. The opposite had been true in that we had pleaded with him to go back to the nursing home because we knew it would cause trouble. But he wanted to stay with us and we let him.
However, the son does have the power to have his father sectioned and held so we did not want that to happen to the old gentleman and so the next day we took him back.
Maybe we were weak but I think sectioning would have broken him, and we knew how they could lie like Olympic lying champions.