Update: Successful resolution!
My sister went for dinner at another bridesmaid's parents' house. The bride and the other Indian bridesmaid were there as well.
The bride made a comment about how she wasn't too excited about having everyone staring at her and being the center of attention. My sister jokingly said "Oh so that's why you are having us wear saris - you want everyone distracted" Table cracks up (bride included) and the bride then said that she thought it would look cool, but that they don't have to wear saris if they won't be comfortable.
So really all it took was a slight hint that people would be looking at them. Gotta love a friend who won't put her friends through something that she herself wouldn't enjoy