Make a date with your partner to discuss beforehand how to survive the holidays together. Be prepared with suggestions and an open mind. Know your priorities. Work out exactly how and when you are handling both families without going insane. Short and Sweet.
Survival is the key word here.
If it's always a nightmare and you can't get out of the family plans, minimize your time with them - actually make other plans so that you can leave early or arrive late. You don't have to spend the entire holiday with them. The less time you have to be on your best behavior the better. Make a list of the positive reasons to go and spend the holiday with your family. Pull out the list whenever you hit a rocky moment.