I went to an Indian wedding and the saris were absolutely amazing ! And as only 1 of 7 Caucasian guests (out of over 200 people) I did feel, in my regular style of dress for a wedding (nice dress & heels), I stuck out, so I feel for your sister. I think the bride is not thinking of anything but how she wants her day to look, but it comes off as rude and IMO slightly racist. My good friend is caucasian, but had an african american girl in her wedding..she didn't make her dress in ethnic garb because of it, she wore the same dress as the other BM's. I think your sister needs to sit her friend down and tell her how she feels about this strange request (using the points you listed in the OP), and if friend insists on a 'look' more than how she is going to make 2 people feel like outcasts, then your sister and the other girl should bow out because it just sounds so darn awkward !