Thanks, everybody, for your input.
I really was on the fence here and I honestly took the advice about "Just say No." The most important advice I took into consideration was the thought of inconveniencing my other guests by making the gathering uncomfortably crowded, using paper plates, etc. All of your comments made a lot of sense to me. In fact I even drafted an e-mail to BIL because I strongly agreed with all of you.
I think there were two factors in making my decision . . .
First, the back of my mind I had a niggling thought of MIL saying something like "Oh, you should have invited them. It's been years since I've seen Jack and Jill! We would have made it work." Actually, MIL has too much grace and probably wouldn't have said that out loud but it was my own self-guilt-trippy thing. I love MIL and enjoy doing special things for her (she's almost 80.) I think that Jack and Jill coming would be a wonderful surprise for her.
Second, and really the deciding factor, is that due to the weather I was toying with the idea of renting a tent anyway. Didn't really want to spend the extra money, but even without the extra people this probably would have been a good idea . . . gives people more roaming room. So I bit the bullet and rented a tent, an extra table and chairs. Doing this really put my mind at ease.
Back-n-forth e-mails between me and BIL: There will only be 6 extra people (BIL, SIL, younger DD & BF, and Jack & Jill). BIL said that SIL will be bringing a bunch of appetizers and I asked if they could bring plates and flatware (especially steak knives.)
So, I think it's all going to work out and everybody will be happy.
Just had to add another reason for me wanting to please everybody:
Just because BIL has suddenly realized that your party is going to be the happening party does not give him the right to gate-crash at the last minute.
This is the
happening party of the year!
This is an annual event (horseradish, anyone?
) and I'm betting that's why BIL asked if it would be okay for him to bring his guests. When he actually called me with his regrets (instead of our usual e-mails) he sounded very disappointed about having to miss this event.
Again, I thank everybody for your thoughts and advice. Even though I went against most of what you all posted, reading your words helped me decide . . . because BIL doesn't usually have a rude bone in his body.