Didn't you have someone in the US who said, "Just say no"?
You'll be surprised how good it feels, and if your husband kicks up a row, let him go there on his own for once- it'd be the last time!
simply decline and thank her anyways. you dont need to explain yourself. just say its not a good time.
JUST SAY NO!.....You are not a kid anymore!
Tell your mother you have to live now!
tell her you heard this from a florida boy. it gets so HOT and HUMID down here in florida in the summer, you don't even want to go outside. and it rains every day. if you want to come down here when it's a true paradise, october and november are florida's best kept secret.
Suggest to your husband that you and the kids do something different this year. If you guys can afford to feed all those people, surely, you can afford an alternative vacation minus MIL, BILs and their wives and kids. Yea, a free trip is cool and all but at the expense of you and your husband's relationship, it is not. Once hubby agrees (make it someplace that would peak his interest), then explaining it to MIL should be a breeze..."Oh, I wanted to let you know before you made any plans over the summer this year, we're taking a trip to ________ with the kids. I know you enjoy having us and we enjoy your company as well...maybe next time. Thank you for offering."
Well your husband seems to WANT to go.. he is telling you that he has no problem with it because it is a "free" place to go each year. Maybe if your husband really wants to go this upcoming year, you should express that you would rather save up and go somewhere with just your family... if he persists on your MIL's timeshare than ONLY agree IF you are able to vacation with your kids while he does all the work (buying, preparing and cleaning up the food).... THEN maybe he will decide to vacation somewhere else OR TRY to do it all himself and realize that the week vacation at the timeshare is not worth all the work -- it is vacation right?? Just do the whole reverse psychology thing and hopefully it works!!!