I was 'chief' bridesmaid at my best friend's wedding two years ago... I still carried my baby weight plus some cake weight and wasn't confident at all.
I LOVED it. I was realistic, I was the fattest bridesmaid there (size 20), the rest were probably a size 14 between all 3 of them!
But I was CHIEF. I had the stature, I had the title, and no one would dare argue.
I also had a very understanding bride who allowed me a slighlty differnet dress design than the others, no movement on colour, but at least I could wear something that didn't make me look like a constipated colon.
YOU WILL NOT LOOK STUPID. Or rather, you will ONLY look stupid if a) your face says you feel it and b) you're wearing something only a size 0 could look good in.
Please don't turn her down, but DO discuss your worries with her. TELL her that you'll need to find a dress you feel happy in, tell her your worries, and then tell her how utterly touched you are to be held in such high esteem.
If all else fails sweety, and this may hurt, you really can lose a few lbs between now and then... I didn't believe I could so didn't, but having lost 2 stone since March (and that's done slowly) I can safely say that if you're not happy the way you are (and you have every right to feel happy the way you are) then you can do something about it with the right support.
Please don't deny yourself this opportunity to be at your sister's side. I talked myself out of being ASKED to be MY sister's bridesmaid (she thought by asking I'd feel awkward, she was right) and I regret it massively.
YANBU to feel the way you do. YABU to dismiss the chance because of the way you feel. x