What makes your sister in law think she has the right to put you down? Why does she have to pick on you, when you've never done a thing to her?
Your sister in law, who never thought you were good enough to marry her brother, snubs you at every opportunity, saying things like "You have gained a pound or 2, haven't you," or "What a lovely dress, did you get that on sale? It looks like last year's fashion." She carries herself as if she's far better than you, is overtly condescending, and in general makes you want to scream! When you complain to your husband, he laughs and says "Oh, hey – don't take it so seriously, that's just the way she is." Argh, no help there.
How do you deal with her?
Take a deeper look. If your sister-in-law's so busy checking you out and putting you down, it's because somewhere deep inside herself, she's insecure. She's afraid you'll get too much attention, or too much of her brother's love. Or perhaps she's afraid that if anyone looks past her stylish makeup they'll find there's not much there.
Forgive her her insecurity, and learn to deflect her put-downs. Next time she says "Gained a pound or two?" come back with "How nice of you to notice how I look – actually I've lost a couple of pounds." If she says your dress is outdated, say "Isn't it great how this retro look is in? " Without getting mean, let her know you're on to her – that she can't bring you down anymore.