I think you should stay out of it, I know it's hard, but leave it to him and his dad to figure it out. They may had some unwritten agreement that you are not aware of, like your h is paying 2/3 of the mortgage but gets to live in the house and you FIL maintains his own residence, so he is only paying a 1/3.
If you want to get that technical, he can charge you rent for the time you H lived in the house.