Have read through all of the above, and completly agree with all that has been said. But just to put another slant on it, what would happen, if your B and SIL LOST THEIR HOUSE, through reposession, or illness or some other reason. Then the other two would be forced out.
There is no easy answer to this, except fot the obvious which involves confrontation and standoff. I know it would be horrible for a short while, but would reslove the situation.
Good luck with your visit.
Lovely Lady