I made the decision to let the women in the wedding party choose whatever they want as long as it fit the color and theme (does not even have to be a dress, could be a jumpsuit, lady tux, etc.). I don't want anyone to have to wear anything or spend anything that would make them uncomfortable.
That being said, I think my future SIL stepped over the line. She and I have always had a bad relationship, and on top of that, she is much (much!) richer than me or anyone else in the wedding party. After bitching and moaning about how she did not like the (extremely flexible??) wedding party attire directives, she finally got a dress. It is a ridiculously lavish $10,000 ballgown. That is not just more than my wedding dress cost - it's more than my wedding dress, my fiance's tux and probably the rest of the wedding party's attire combined.
When I mentioned that I thought it was little expensive, my future MIL (who bought the dress for her) called my fiance crying about how I am ruining the whole wedding. I buckled and said SIL could wear it.
It's still bugging me, though. Like really, really bugging me. She has thrown tantrums at every stage of this planning process (she wants to sing a song at the reception, she wants to invite a bunch of her friends) and we have given in at every point in order to avoid more fighting, but she keeps demanding more. Can I ask her to wear something else? Is that a diva move? I honestly can't tell if I am being a 'zilla or a doormat or some weird hybrid.