it would take me aback too if someone just said they were coming to stay, and didn't actually ask how it would work. but i'm not quite clear on how it went down with you.
i mean, if she said 'i've got everything covered but the last night, is it okay if we stay with you for a single night?' then why are you so butthurt? you can say yes or no and carry on.
if she simply said 'we'll be visiting for several days and have to stay with you one night' and didn't give you an out, then yeah, that sucks, but you can still be firm about what you will or will not tolerate.
i'm going to have a full house over christmas and was very sad to have to tell a beloved family member that the usually-available spare bedroom won't be available, and that i don't know how this person is going to work it out. the floor is always an option, of course, but not something everyone will go for.
if you're totally uncomfortable with everyone having a big pajama party for a single night, then of course you courteously and firmly say 'i'm so sorry, thelma, but that's not going to be an option. we'll be bursting at the seams on that date and just can't accommodate your ravening horde. can i help you find a motel, or a cute B&B for that one night?'
i myself would go for it. at these ages the kids are still having a blast with christmas, and one night isn't a huge deal. put the SIL and her partner in your kids' room and let the kids build blanket forts and stuffed animal piles.
but you get to pick. it's your house. if you aren't comfortable using your voice, you can't expect anyone to read your mind.
khairete
S.