Mentally Prepare Yourself
Chances are, a difficult relative isn't going to suddenly come out of the woodwork. If Aunt Ida was a challenge last Christmas, you can expect the same this year, and you know exactly they type of behavior she will pull out of her hat. Go into celebrations and family gatherings with a game plan for how you will deflect certain comments or change the subject when a particular comment or bad joke happens. You'll have a much easier time staying calm and getting out of a sticky situation if you have an idea of how you're going to handle it before hand. You can just stick to the plan as opposed to having to think on your feet.
Have a Partner in Crime
If you and your sister both share the same problem with one relative, make a mutual deal where you'll save each other from an uncomfortable conversation with that person. Knowing that someone else at the party has your back can take a lot of stress off of your shoulders. Your sister (or other designated family member) will swoop in to pull you away if they spot you being hounded about being single for the 3rd Christmas in a row, or dealing with yet another offensive joke from your cousin's husband.
Don't Contribute to the Problem
Just because someone in your family insists on being a thorn in your side doesn't mean that you have to retaliate with bad behavior of your own. While you don't need to be a doormat, don't let yourself get riled up and yell at someone -- because then you'll just look like the bad guy who ruined the Christmas party. If a family member tries to pick a fight, let her know flat out that you're not going to argue on the holiday. If someone insists on chiding you about some aspect of your life, just let him know you don't want to talk about that subject and turn the conversation to something else. Repeat yourself if necessary, or leave the room if that is the only way to diffuse a potential argument.
Try to Empathize
If someone is really negative during the holidays, they usually have something bad going on in their life or are generally an insecure or unhappy person. Try to look at your difficult family member through that lens, as difficult as that may be, and remember that they are communicating in the only way they know how given their state. They might actually be trying to connect with you even though it seems they're being critical or harsh.
Sometimes taking a few minutes alone in a private room, or going for a walk, is all you need to calm down and re-energize. These 10 to 15 minute breaks will give you a reprieve from your troublesome family members and give you a chance to consciously focus on something positive. Use this time to call a friend who is somewhere else and make jokes to get your mind back in a positive space before you return to your family.
The family members in your life might not always be the easiest pill to swallow, but just remember that in a matter of days you will return to your regular life. Try to focus on positive things about your family and turn their flaws into funny quirks in your mind.