The theme of the wedding is the responsibility of the couple and their attendants, not of the guests. Not even the sister of the groom .
So wear a nice dress, one suitable for day, maybe a little bit fancier. As in, a dress versus separates, that sort of "fancier."
You won't need to wear something sleek or shiny, since it's not evening and it's not a formal afternoon event.
I vote the B&W floral SL Fashions one, not the lace one. And the bold-print sheath would be OK if you personally like it.