Ooohhh, Jinkies. Tough subject.
I, too, have purposely avoided people because I've gotten so fat. I especially do it when I go home - I try very hard not to frequent places where I might run into people I knew in high school. (Isn't that pitiful?) I also have a friend here that I haven't seen in about a year because I have gained about 50 pounds since the last time we saw each other. Unfortunately, it's too easy to use the excuse of "too busy."
The only advice I could offer with your family (and it's so much different with family that it is with friends) is to beat them to the punch. Make a funny, self-deprecating comment before they get a chance to look shocked or surprised. (For example, a couple of years ago I'd gone home about 30 lbs. heavier, and my aunt and I were doing dishes in the kitchen after a family dinner. We kept bumping into each other, so I said something to the effect of "I think my hips are just two too many!")
Another suggestion? Look your very best. Dress to the nines. (Unless you're like me and can really only afford to dress to the eights...
) Be meticulous. Get your hair and nails done before you go. If you feel better about yourself, you will carry yourself with more confidence and others will pick up on it.
I wish you the very best of luck. I truly know how you feel.