In reality most of us have real families with flaws and foibles, and chances are if we didn't get along with relatives before, the holidays won't set things straight. You won't change your family over the holidays, so lessen your own stress by just accepting them as they are and scheduling visits in manageable chunks for your sanity. It's ok to take a break from your regular exercise routine over break but try to get some physical activity daily. Making a complete conversion to couch potato can result in a significant decrease in cardiorespiratory fitness levels in only 1-2 weeks. Taking a walk after meals and helping with daily chores are beneficial to your health and provide needed breaks from the mall or couch.
Physical activity is a great mood enhancer as well.
Take advantage of extra time and start a new exercise, stress reduction, or recreation routine. In terms of holiday traditions, think about letting go of your stranglehold on those childhood memories. Don't feel the need to retain every holiday tradition of your youth. If it becomes more stressful than enjoyable to recapture the rituals of yore, consider letting them go. The best way to avoid a nutrition meltdown is to plan well and recognize that you'll eat differently during the holidays.