Your other sis is a fucking knobber by the way... I know you love her an all, being your sister, but HOLY MOLY... she needs to be tripped up by a certain CHIEF bridesmaid on the wedding day doesn't she? (on the way out of the church/office not into)
You have what, almost a year. Get theeself to slimming world (evangelical moment sorry, I am not sorry) and eat yourself thinner.
And meh, you know what? Lose weight, don't lose weight WHATEVER... don't lose heart, your (nice) sister chose you as her 'matron/chief' and not your bitch of the other sister (who's probably nice deep down but I'm very forgiving, I actually think Ursula has many marvelous qualities and is down right sexeh) so listen lady... sort it out. Be proud of who you are, be proud of what you look like, make sure you look darn good... whatever your size, and if you're not happy, you have quite a few people on here who will happily be a buddy to you whilst to get to happyville (whether that means finding the right dress for you, or finding the right you for the dress).
I missed my chance with my sister because I didn't like myself. I'm by no means skinny now by the way, you won't find me in some magazine telling you about 'my story' with a glam size 10 photshoot... but pffft. I'm bloody ace me, I feel great, I'll stand next to anyone and be proud, whther they're a size 10, 0 or 42, and I'll feel OKAY!
Chin up. You are lovely. AND I don't even know what you look like!