I think you would be doing the right thing Mila...Its for you FIL, not your H. BUt I sure would make it known that the OW will not be allowed for a visit. Hopefully the inlaws would be visiting your FIL most of the time they are there and hopefully they would just be sleeping at your home for the most part.
I have thought about this myself before. MY H's mother lives out of state and I know if she wanted to visit, I would let her stay with me. H could visit her if he wanted to. BUT my H knows there will be NO OW in my house at anytime whatsoever...that is MY home!
Kissak
"What time I am afraid, I will trust in thee." Psalms 56:3 M-37 H-37 S-10, D-15 M- 1993 First bomb- 12/23/06 Came and went too MANY times! Gone again 10-25-10