Limit the length of the trip. The extended family might be gathering for the week, but you and your school-age children might do better to join in for just a long weekend. Have good reasons for going. Schools are less likely to be punitive if the trip is for a good reason. My childrens' teachers have generally been sensitive to the issues of our restricted vacation time, perhaps more so than if we'd decided to travel simply to miss the crowds.
Overseas excursions are wonderful and valid educational opportunities.
Fitting in a side trip to a place pertinent to the lesson plan is bound to be appreciated, especially if you offer to have the child keep a journal during the trip and perhaps write a report of the experience. Take school work with you. Although it's not very vacation-like, educators appreciate the effort of keeping up with school work while away. And if you can't get the work in advance, enlist someone to help you. Lauren got around school rules by hiring a teenager to pick up the daily assignments and fax them to her on location.