What should I wear to my sister's wedding?

So, as I may have already mentioned, my sister is getting married in DC next week. This is the second wedding for both her and her fiance, and it will not be a big, traditional affair. Only immediate family (parents, siblings, and children) will be there, and there is no bridal party or anything like that. There will be no big white dress. In fact, my sister is wearing a knee-length purple dress. (She likes purple as much as I do.)

Her dress is fancy, though, fancier than anything I own, so she will definitely stand out. So it's not like I have to stay away from purple because that's the bride's color. In fact, as far as we're both concerned, the more purple the better.

Of course, my options are limited, since the vast majority of my formal dresses are way too big by now. In fact, I have only three options, which certainly makes picking a dress easier. Luckily, two of them are purple.

Anyway! Let's look at the three dresses, shall we? I'm leaning strongly toward one of them, but I definitely want to hear opinions from others.

Dress A (some longtime readers may remember that I wore this dress to my rehearsal dinner--which was on Halloween, in case it isn't obvious from the photo--when I was significantly heavier, but apparently it's very flexible with its sizing, because it still fits):


Dress B:


Dress C:


So what do you think? Which one should I wear?