I had a long talk with dh about this late last night. He told me that if she really wants to go and we tell her the van (12 person) cost too much, that his mother will feel bad and she will definately pay for it. Mil can't go because she doesn't like to travel, she has 9 animals of her own and would also be sitting for sil 4 dogs, and checking in on sil 7 cats. (I must say though they are obsessive cleaners and sil is a groomer, their homes are immaculate and their animals are always clean.... you'd never know they each had so many pets).
Anyway, dh made it clear that his sister expects us to do everything together. He says that she will see it as a vacation for the kids, not her and want to keep them together. I can kinda see a point, I mean dd will be 5 and her cousin is 6 and they are best buds... I don't think dd would be agreeable to a split up. Plus, I like hanging out with my 13 and 11 year old nieces (they'll be 14 and 12 during the trip)... frankly I'd hitch up with them and meet up with the rest later, I know the 3 of us would have a blast. Dh said that there's adult things at night that he wanted to do, that he and I could do together if sil came too. Dd, dh, and sil will celebrate birthdays while on the trip so it should be memorable for all.
We'll probably do Disney again next year or the following year. So the plan we came up with is, we'll tell her the van is too expensive. If she really didn't want to go that will be the end of it. If she really wants to go then dh is sure his mommy will pay (I talked myself into them coming, so I think I may have dh work on his mom from the other end). If she changes her mind last minute, it's not on my dime... we'll just go along with our original plans. If they come and I experience too much togetherness, dh and I will ditch them at the house and drink our frustrations away! Either way, I think it might be ok... I'll have to let you know on the post-trip boards. Thanks for the great advice!