Everyone has replied with some outrageously great advice. Can't add anything but the steely glare option when a less than kind comment is made...Jackie O used this technique quite often I read...really just put people in their place without a word.

Know that we're here to support you and think you are wonderful. Anyway, if you just can't get out of this soiree, might you be able to go out and get yourself something fabulous that you have been wanting for a while and putting off getting?

(Short of that, perhaps we can get some dashing young handsome Special Ops team members to come in and rescue you at the preciously the right moment that will daze your auntie and sis beyond words. I know that this can be done...just got to find that secret cell phone number to The Prez ).

You've gotten some excellent suggestions but I'd just like to add that there are some people who try to make themselves feel better by putting others down. I have a sister-in-law like that and believe me I know how you feel.
If it's not absolutely necessary to see your aunt then why bother? I don't see the point in being around others who try to make you feel bad about yourself just so they can feel good.
If on the other hand you have to see her then just smile even if she says something negative. You can't be faulted for smiling and it will make her wonder what you're up to. I think it's best not to get defensive cause if you do the person knows they got to you. Best wishes

I cant get over how kind and supportive everyone is here....your responses truly mean the world to me and I am printing this thread out so I can keep it with me....our visit got postponed till next week (she wasnt feeling well) so hopefully I can get DH to take a few pics and get opinions on what looks best (not just for this, but in general.) Sorry it took so long to get back on here, but DH has been home from work and getting online for more than a minute or two is just about impossible...

1.) Wear something you feel fab in
2.) Be confident!
3.) Congratulate your sister and tell her how great she looks
4.) Tell auntie how happy you are to see her
5.) And know that all of your YLF friends are cheering you on!!!

Enjoy yourself....

Oh, I thought of another response. I just read Ask Amy (that newspaper column) where someone wrote about gossiping and how a good response is to say to the gossiper, "You know, she/he speaks so highly of you."

Along those lines, I suggest a sweet smile and these words: "Oh, auntie/sis, it's good to know I can always count on you to say the right thing."

Oh Biscuitsmom, do I ever understand what you're dealing with!! I have family members who comment continually on my weight. With them it's done out of love and concern rather than malice, but that knowledge doesn't help, and the comments cut to the quick.
Line up a support network. We're all here, obviously, and I'm sure your husband is too. Knowing there others to make you feel better will make it easier to brave the temporary unpleasantness.
Wear something that makes you feel comfortable. I cannot stress this enough. By this I don't mean physically comfortable, but something that feels like you and will allow you to relax as much as possible. I need to take my own advice here, honestly, and I can tell you from experience that sacrificing your own preferences in an attempt to please others doesn't do much good.
Look polished on the day of. It helps.
Take Biscuit. Dogs are amazing helpers, as you already know. Plus, walking him could give you an excuse to get out of there!
Don't rise. I like the idea of congratulating your sister on her own weight loss and generally not buying into the negativity around you.
POST PICTURES!!! If you put together an outfit with YLF input, I guarantee you'll feel better about it. It'll be even easier to picture us cheering you on!