My sister hates dresses! And wants to be a bridesmaid

Hmm, I think that more information is needed as to why she is uncomfortable wearing a dress. Just because someone was willing and comfortable wearing a dress in the past doesn't automatically mean that they're comfortable wearing one now, and to all the PPs saying that this person is just being "difficult," well, this isn't really fair without all of the info. My cousin, for example, wore a frilly bridesmaid dress for her older brother's wedding. After she came out a year later, she no longer feels comfortable wearing dresses at all and will only wear pant suits to fancy events (she then wore a pant suit as a bridesmaid for younger brother's wedding, which looked fantastic by the way because the color matched the rest of the bridal party). I think it would be very important to know why she is saying she'd be uncomfortable in a dress.

It sounds to me that your sister wants the attention that comes with being a bridesmaid without the flexibility, too. I have friends who also don't like dresses, so I made sure not to ask them. I think in the long run, I did them a favor. I may be a little biased because I'm actually kind of done with the whole pantsuit thing, but as I said to another bride on a similar forum, YOU are the bride. The bridesmaids are the people you're supposed to trust the most, and with that trust comes trusting your judgment. If your sister really cared about you, she'd do the decent thing and wear a dress for a day. I honestly don't see how it's really going to put her out that much. She can still wear pants to the shower, bachelorette party, and rehearsal dinner. That seems like a reasonable compromise. If she won't ease up, then it might not be a bad idea to think about backup bridesmaids and think of another job for your sister.

Likewise. If she could make an exception for her own wedding, how is it so much more of a task to do the same for not only someone else, but her sister?