I don't know much about Russian Orthodox, but I went to a wedding on Saturday. Most girls were wearing pretty dresses from places like Debenhams (mine was from Jane Norman) with sandals and shawls. Terri, the groom's sister who didn't want to be a bridesmaid, was an usher, and she wore a smart trouser suit with a corsage and had her hair done nicely.
But this was a Roman Catholic wedding. You're probably better off asking someone in the family, or better still, why not get in touch with your brother's fiance and ask what you should wear? If you're one of the main witnesses then your outfit will matter to her, so she'll be pleased you got in touch. But do it soon. Once the wedding gets near they'll all be stressed out with all the last minute arrangements.