Hey, it’s the 21st century - it happens. Sometimes a man’s best friend is a woman (next to his wife I’m sure), and he wants this woman to stand up for him at his wedding. Other times the best person is the groom’s sister (particularly suitable when the groom and best person are twins).
The role for the best person is essentially identical to the role of the best man. Why change things? The only consideration that may be discussed, depending on the circumstances of the wedding party, is best person attire. So what is the proper attire? Well, for the most part, the groom, best man and ushers wear tuxedos for a formal wedding. This, however, does not mean that a tuxedo is mandatory attire if the best man is female, and in most cases it looks inappropriate.
Several sources have suggested that the best person should be attired in colors similar to that of the groom and ushers, except, of course, wearing a dress. The dress should be simple yet formal and should in no way conflict with or upstage any other dress in the bridal party, particularly the bride’s. Black is usually the most common and accepted color, and should be the color of choice, wearing black nylons or stockings and black shoes. Nothing should be worn in the hair (flowers etc.) and any flashy jewelry should be avoided.
If the groom is wearing white, usually the ushers will wear gray, so the dress should be gray as well. The whole point is to blend into the wedding party, not to contrast with it, although what the etiquette is in these types of situations tends to vary. The best person should consult the bride and groom in regards to these matters, as often times a color scheme can be, and is discussed.
As you can plainly see, the responsibilities of the best man/ person are fairly straightforward, although they do, and can, differ drastically from wedding to wedding. The key is to know ahead of time what is expected of you - that way there are seldom any surprises that will pop up to haunt you later.
Another key factor in undertaking any of these duties is to be prepared. It certainly doesn’t hurt to read up on things, or to ask other friends for advice (providing that they have some to offer), and having done this, your friend and his bride will be eternally grateful.