Yes - I did start to show much earlier with #2 - simply as the uterus has stretched before so instead of holding baby taut inside it just relaxes outwards!! It all evens out in the end though....
Having said that, I would not put fitting into a bridesmaid dress above my desire to TTC. Every month is a precious oppurtunity and it can take longer than you would like. My cheif bridesmaid was 6 months pregnant at my wedding and we just modified the dress for her lovely bump. I have also been to plenty of weddings where there is a pregnant bridesmaid. IMO your SIL should be willing to make allowances for you and be happy for you. Sure, her wedding will be her day - but its only one day and I don't think its fair to be expected to put your life on hold for someone else's.

Best of luck.

X