@Imogenlily , I obviously don't fully know your relationship with your fil and sil. But from what I am reading it is not just the ss that is taking advantage of you. I wonder if you should not be focusing on the ss, but rather your sil, to get your fil out of your house. You mention building work. Perhaps that is a wonderful opportunity to lose a number of essential services which requires your fil to leave the house for his own comfort and safety. You may need to camp for a bit, or look ad if you are when certain people come visiting.
But as I say I don't know all of the relationships involved.
Good luck.
But as I say I don't know all of the relationships involved.
Good luck.