If I were you, I would go just as you look in the pictures, and just keep a low profile. I mean, you are looking feminine, that doesn't really attract lots of attention from a casual glance. Of course, there might be a question or two, but if you don't volunteer information, I assume that people will probably talk behind your back before they talk to your face, at least until they drink. It is your sisters big day, but I wouldn't assume that going dressed like that is going to "steal the show".
Ofc, if you have an active role in the wedding that might make things a bit more complicated. Still, I wouldn't change, and I probably wouldn't say anything before hand because if it were put into words, it would almost have to sound much weirder than it is. I mean you look normal, and if a group of people who didn't know you seen you, they wouldn't look twice. I have found though, when I was trying to explain to people that I was going to start wearing proper female clothes, they apparently had very fanciful ideas about my looking like someone who did drag.....So talking about it might send the wrong signal.