I was just a bridesmaid a few weeks ago with my EBF 3 month old. A pain in the butt, sure, but it worked out. I think straps will be a nuisance, honestly. The dress I wore was strapless as well and they make them with so much boning that there was plenty of support without a bra. I just bought some nursing pads with sticky strips and stuck them on the inside of the dress. To nurse, I just draped a nice big Aiden and Anais blanket over my shoulder and flipped the top of the dress down. Certainly not as comfy as nursing at home, but it was only for a day. Then halfway through the reception I changed into something more nursing friendly.
My advice: don't wear a bra, buy some sticky nursing pads (Lansinoh are great), get a generous sized nursing cover or blanket, and ask if you can change once pictures are done. As far as sizing goes, I ordered mine when I was 7 months pregnant and they were very good about figuring out the size. Don't worry about it. You will regret not being in your brothers wedding though.
And about a 21 month old flower girl, don't worry about that either. No matter what she does, people will think its cute!