Now, if it's the Mannings, you are referring to both siblings. Williams is the actual name of both Serena and Venus. Ergo, for the sake of continuity, the correct form would be Williamses, the plural of Williams. Yes, everyone is used to "Williams" from "Williams sisters," and Williamses looks clunky. But it's also "Manning brothers," not "Mannings brothers." So this is a definite fail, albeit probably not a very important one in the scheme of things.
