J, It's not clear to me what you want to do. Are you looking for how you could graciously host them on paper plates and not enough seating or are you looking for the polite way to say no?
Thanks, arila, for asking me to clarify my muddled thoughts.
Originally I posted for a polite way to say "No." Mostly due to the last minute (rude?) question of "can we all come" along with the weather forecast and my inside accommodations.
The more I think about it -- I'm leaning towards finding a gracious way to host on paper plates without enough seating for everyone.
MIL is coming from out of state. I'm betting this is why BIL asked . . . he didn't know that his mother was planning on coming and it's been 6+ months since he's seen her. MIL will come up on Saturday morning, stay for the party, sleep at SIL's home then breakfast with old neighbors, attend a church function and leave Monday morning. So there really isn't a time, other than the party, that BIL can see his mother . . . Jack and Jill are just a side benefit at this point.
So, I'm still on the fence . . . I do agree with everybody that BIL was rude about asking, but I do see his point and would like to accommodate.
Food-wise? I always make plenty so adding a bit more to the side dishes won't be a huge deal.
Serving on paper plates? Not such a paper-plate-friendly type of meal. Would I be a rude hostess if I had regular plates along with some paper plates (I do have the paper plate holder thingy's) on the buffet table? What about the flatware? Mix plastic with the real thing?
What about seating?
Sigh. I really want to accommodate everybody, but am not sure how to do this without being a rude hostess . . . I need to give BIL an answer tomorrow.