Thanks to everyone for their input.
I think my biggest concern is that we might feel a little uncomfortable with a "stranger" in our midst.
As to the "sleeping" concerns, rest assured that they would not be sleeping together out-of-wedlock

under my roof

. That would not be tolerated with my mother and children present. (According to her that is not even an issue [at this time].) I had planned to either put DS11 in our room on the floor, while mom, DN and DD shared the studio portion, and BF was on the couch, or put the 4 of us in the studio and mom and DN in the king with BF on the couch. Without him, it will probably be mom, DN and DD in the studio with DS on the couch.
As for the niece imposing, yes she probably is, however we are extremely close. While she was young and we were childless, we traveled with her extensively. Her parents don't travel, and we wanted her to experience more of the country. So her complaints, were more jokes. She truly feels she is our third child. She has accompanied us to Florida, Missouri, Kentucky, Tennessee, Hawaii, etc. When we drove, $ was not a big deal. When we flew, my mom normally picked up the airfare. This time she was told she was on her own for $. She is responsible for her airfare, ticket, food, souv., etc.
I think I may discuss whether he would be amenable to paying $400 for a studio at OKW. I think I would then try to find points for him to rent. We don't have enough to do that without borrowing, and I don't want to start that yet. Nevertheless, DN would be expected to sleep in our room. That is not negotiable with my kids present.
Maybe I could get a night of babysitting out of them (if he agrees to a separate studio) so DH and I can go to PI

.
Once again, thanks for all of your input, and thanks so much for keeping it civilized. That is why I posted here and not on the other boards!