Well.....having been on the bride end of things recently and the bridesmaid side plenty of times too, once you agree to be in the wedding (which from the sounds of it, you weren't even asked, which royally stinks....) it's pretty much "what the bride says goes" and buying the dress/shoes/hair/makeup is usually taken on by the bridesmaid. After researching wedding etiquette for a long time....guys on the bride's side, different colored dresses, different lengths, shades, styles, anything goes! I've been on the receiving end of awful dresses (lavendar?? I agree with you on the plum/aubergine....!) and decided for my wedding to ask the bridesmaids to have as much input as possible and asked them about styles/cost/etc. all the way through (the dresses were black, two-piece, tea-length and 2 of the girls have already worn their dresses to other weddings, which totally makes me happy!).
Unfortunately, I don't think it happens that way in most weddings, but if you're close to your sister, maybe you could go shopping with her for dresses? Maybe suggest/try on dresses in other colors (or variations of what she thinks she wants)? And/or send her a few swatches of wedding color suggestions (try Martha Stewart, Real Simple, Ann Taylor (has a gorgeous midnight blue color out right now), The Knot, etc.). I hope you guys can come to an agreement!!
It can get to be a sore subject even in the best of situations with all the general stress of wedding planning....I hope it all works out for the best! (And show us some pics!).
T.