I recently took a two and half day trip with my family to New York. It was just the original clan -- no boyfriends, husbands, or kids. I had a great time, as I always do when I'm with my family. It was a rare opportunity to get to spend more than just a couple of hours at a time with the people I love most in the world.
The risk of being around family for an extended period of time is that these are the people around which you're the most comfortable -- the most able to be yourself, warts and all; therefore, they're the people around which you're the least "conscious." It is so easy to revert back to old (bad) habits and familiar (albeit dysfunctional) patterns of interaction. If you're not careful you'll find yourself feeling jealous of the older or younger sister who was always a little prettier or smarter. You'll catch yourself competing with beloved siblings for your parents' attention or snidely dismissing something a sibling or parent says as stupid or closed-minded (when really it's you being stupid and closed-minded).
You'll catch yourself becoming inexplicably annoyed by a parent or sibling's harmless habit or asserted point of view, one which he or she has always held and that you'd previously come to accept a long time ago, although you don't agree with it.
Hopefully, you're conscious enough to realize you're doing these things. You could ask yourself why but the answer is probably rooted in complex family dynamics that would take a professional to unravel and interpret.
The thing to keep in mind is that these are the people you love the most in the world -- the people you would do anything for -- and the people who love and accept you no matter what you do. They love you even when they find themselves annoyed by your obnoxious tendency to criticize their opinions or your petty sibling rivalry. They love you and they accept you. There's no need to beat yourself up for slipping into old patterns of behavior when you're around your family.
The hope is that you'll become more aware of it and make attempts to alter your behavior, little by little. This enables you to establish relationships with your family as people -- as you move beyond entrenched patterns of interaction you get to know your family members as unique individuals as opposed to viewing them only in relationship to yourself. It's this kind of growth that nourishes and strengthens families.