while i'd be annoyed that my husband agreed to something he doesn't actually plan on following through with (that's pretty blatant dishonesty, something i don't hold with at all), i'm not sure why your knickers are so twisted over it. i mean, he's made it clear to you that you're not going to have to deal with it, right?
or does he lie to you as easily as he does to his mother?
i'm a hermit, and have quit doing the Big Family Vacations altogether and am glad of it. but when the boys were small we did do some, with both sets of families. for me it was a small price to pay to foster a close, warm relationship between my kids and their extended families. and i'm glad we had families who wanted to do it. and even though i found it stressful, i managed to have fun also.
now, maybe your anxiety is of a much higher degree and it's really something you just can't do. no fault of yours. but there are much calmer, quieter, happier ways of handling it that being livid and so upset and suffocated and torqued.
you've got a MIL who cares enough about you all to want to spend time with you. doesn't mean you have to accede to every request, but you could at least be gracious about it, and find honest non-hurtful ways of including her into your family without 'suffocating.' having a meal with them every week is terrific. good for you!
you get to pick the degree of your involvement. if family is important enough for you to endure some discomfort, you can all go to Great Wolf and then scatter. even on family vacations where we were all packed into close quarters we made it very clear that we would spend some time- sometimes most of the day- with just us and our kids. but it was nice to have a daily planned activity or a meal with the rest of the bunch. or for the cousins to all go do something with one set of parents while the others enjoyed some kid-free time. or dump all the kids on grandma and grandpa and all the parents enjoy cocktail hour. there are endless configurations to make it do-able.
if even that degree of closeness is unbearable, then you set your boundaries courteously, kindly and clearly. 'we love you very much and are so glad we all love to spend time together. we want this vacation to be a close bonding time for just us and the kids so we're not going to include anyone else this time around. but the carnival will be in town next month- can we plan to spend the afternoon and evening there?'
and while i myself would have issues with a husband who came out that glibly with a lie, i think you should let him somewhat off the hook, as he's caught in a tough place between his mom, who is his mom and presumably much loved by him, and his wife whose anxiety issues make it hard for him to keep both beloved parties happy. you can make it easier for him by not coming unglued at his coping techniques, which are clumsy at best. but instead of just being furious with him, help him create a solution that works.
khairete
S.