Controlling, meddelsome in-laws want to crash our vacation... Help!?

Living overseas, away from family and friends hasn't been easy. Having overbearing, imposing in-laws who put constant pressure on and pre-plan all of our free time has made it almost unbearable. However, for the sake of my daughter and husband's happiness, I have tried as much as possible to be positive and accomodating if only to keep the peace. However, I'm thinking I might blow. I have just found out, after planning a lovely 2 week pressure free vacation (the first for my husband and I alone without any family, and the very first with our baby) for which I have been super excited for, that my in-laws are trying to take time off work so that they can join us... What what what???? Who invited them? All of our free time seems to be theirs... how can my wonderful vacation be theirs too? I want to be respectful and polite but how do I avoid this? I'm no longer excited to go and actually dread the thought.