And I totally disagree you. You think she discusses it with her husband every time she leaves him with her kid? They are a family and those kids should be welcome at all times no matter what. It's their house, too. To think or say otherwise is sickening. I feel sorry for those poor kids.
Her DH had no right to dictate how she was to spend HER vacation, regardless of who's kids they were. If I worked full time, and took a vacation week, and my DH told me he cancelled our sitter, and the kids would be with me the whole week, without previously discussing it with me, he'd be taking a week off, too.
I know people who "share custody" of their parents with their siblings. So, would it be okay if your spouse's sister decided to go away on vacation during her time, and the IL's were in your care for a week that you planned on doing other things? Without even asking you?
If DH took a week's vacation, there is no way I'd be like "oh good - I'm heading out of town - they're all yours for the week." Issues like these get discussed in our home. With as many kids as we have, there are many times someone has to sacrifice his or her plans - however, DH would never tell me what I had to do.
But I do understand a little of how the OP might be feeling. 

Oh wait, are you serious? 

