It's crucial to answer these questions before, not during, a family gathering.
Prior to the event, think through various boundary options until you come up with a scenario that makes you feel comfortable. Would you be more enthusiastic about a get-together if you planned to leave after no more than four hours? Or three? Two? One? Would you breathe easier if you rented a car so that you could get away without relying on relatives for transportation? Would it help to have a friend call you on your cell phone halfway through the evening, providing an excuse for a graceful exit?
Once you've set limits that allow you to feel emotionally safe, you'll find that your relatives seem much more lovable and enjoyable.