The Mitford girls were probably the most spectacular sister act of the twentieth century.'' ''Vogue'' This is the story of a close, loving family splintered by the violent ideologies of Europe between the wars. Jessica was a Communist; Debo became the Duchess of Devonshire; Nancy was one of the best-selling novelists of her day; the ethereally beautiful Diana was the most hated woman in England; and Unity Valkyrie, born in Swastika, Alaska, would become obsessed with Adolf Hitler.