I think we are all kind of not looiking at the whole situation here. Its all fine and dandy to go on vacations with your inlaws when you dont live with them. But when you are with them 24/7 iw ould imagine you would need a break, just like sometimes we need it with our parents/family. There is nothing wrong with that. And from what i understand, the original poster is looking for a time away from her usual life, with her husband only.
^ I guess from the description, it doesn't sound like a vacation that she and hubby planned that the rest of them intruded upon. Is it? It seems more that everyone was planning it and she doesn't want to accompany them. I do feel that there are reasons she doesn't want to go that are not listed there.
In view of both the explanations of the situation I will advice her to elope with her husband .