By Susan Sheehan, Washington Post Writers Group The Mitford sisters may be the most written-about girl group in history. Nancy, Pamela, Diana, Unity, Jessica and Deborah: The flamboyant British aristocrats have kept tabloid journalists and biographers scribbling away for nearly a century. Between their high-profile trysts, questionable politics and literary output, it’s no wonder they’ve been the source of fascination. Mary S.