Re: What could/should the bride/grooms sisters wear on the mehndi/wedding/walima?
my bm's had yellow and red shalwar kameezes for the mehendi, and my husband's groomsmen had dark green shalwar kameezes.
and then at the wedding, my girls were in identical pale green saris and the boys had tuxes on.
maybe you can pick a color swatch and tell them they can wear whatever, so long as its in that color. that gives them the flexibility to dress up according to their own taste/style, but they're still unified by color.