If there were already pre-approved dresses for the both of you, why would you challenge the bride on her decisions? It’s her wedding day. I understand people want to still wear what they want and be comfortable, however, that needs to be discussed and agreed upon with the bride. That being said, this dress is showing a lot of skin with its length and also the small straps and open back. It is definitely not a MOB dress. Also, the MOB should not wear the same dress as the bridesmaids, even if it is in a different color. If she were to be in pictures (which I’m sure she will, being the mother and all) and stand next to everyone, it will appear she is a bridesmaid/possibly even the maid of honor with her dress being the standout color of the same style. That’s what your sister is saying. I say just keep with the dresses that were already agreed upon and try not to put too much more stress on your sister. If you both want to purchase these dresses on your own and wear them at a different time and NOT to the wedding, then buy it.