Tip: in listing the wedding party, you only really need each person's name. But if you have the space and inclination, it's extra nice to include some context, like "sister of the groom," or "godchild of the bride," or "friend of couple." It goes a long way to helping your guests understand the emotional connections between all the key players. ("Oh, look! Isn't that nice? The 'best woman' is the groom's sister, and the 'man of honor' is the bride's brother!" and "Oh, look, the bride will be accompanied down the aisle by both her parents!")
In the interest of making your little wedding-world a smaller, friendlier, more connected place, it also provides more information for guests to chat and interact during the reception or after-party. ("Oh, you're the best man. Nice to meet you. Did you go to Brown with Adam? I went there, too; class of 2010! What dorm were you in freshman year?") It's all about inclusion of your guests, and consideration of them as an audience. Except it's not just an audience — it's a collection of the people closest to you, to your spouse-to-be, and to your families — it's one big mash-up of "This Is Your Life."