Seriously? So many of you think this is a problem? If the MIL is trying to upstage the bride then that's weird, but quite obviously her issue not the brides IMO. Just ignore! Or, if she's innocently wearing white with no ulterior motives, then it's just a colour!
Why would it matter that it looks like there are two brides in some pictures? It would be a little strange, yes, but it tells a story. It may well be a bit batty on behalf of your MIL (if she does have ulterior motives) but in my opinion that doesn't "ruin" any photos, it adds a story to them. Do you not think you'll be able to smile about it in the future?
Back in the (very) old days you would have had loads of your friends dressed exactly the same as you anyway. From wikipedia:
"The Western bridesmaid tradition is thought to have originated from Roman Law, which required ten witnesses at a wedding in order to outsmart evil spirits (believed to attend marriage ceremonies) by dressing in identical clothing to the bride and groom, so that the evil spirits would not know who was getting married.
Even as late as 19th century England, there was a belief that ill-wishers could administer curses and taint the wedding. In Victorian wedding photographs, for example, the bride and groom are frequently dressed in the same fashion as other members of the bridal party."