oooo, I really feel for you. I am sorry this is happening.

Perhaps you can speak with your F about setting up some boundaries and make some changes re: having guests. Also, important to speak amongst yourselves about some "house rules" .

A day or two here or there sounds reasonable, but extended stays (especially with a seflish, N person) ... even Mother Teresa would have a problem.

Also, I belive it is your F's responsiblity to speak with the SIL and explain the rules of the house (whatever you decide: put dishes in dishwasher, pick up after yourself, use 1 towel, one set of bed linens...here is a quarter, the laundry mat is right around the corner, etc.)

It is your home. If these rules are not respected, then the person is welcome to stay at a hotel.

I know I sound harsh, but I agree with Benjamin Franklin's philosphy on visitors

(ps, if the person was struggling: homeless, jobless, hungry that is a whole different subject altogether. But there should still be house rules. Whatever the case, it's your castle!)