The most fascinating of the portraits in “The Six” is of Diana, the peerless beauty of the family, with her “dynamic serenity, her sphinx smile.” Even in old age Diana’s Grecian profile and hooded sapphire eyes were cool perfection. Thompson wonderfully evokes her coup de foudre with Oswald Mosley. He, like her, was married with no thought of divorce when they met at a society party in 1932. Diana abruptly left her adoring, hugely rich young husband, the brewing heir Bryan Guinness, whom she had defied her parents to marry and with whom she had two small children. With characteristic contempt for others’ opinions, she set herself up alone in a residence on Eaton Square to be near Mosley, whose wife suddenly died in 1933. Three years later, Diana Mitford and Oswald Mosley were married in a small, private ceremony in the Berlin drawing room of Joseph Goebbels. The guest of honor was Adolf Hitler.

Diana remained in Mosley’s dark thrall for the rest of her life, despite his constant infidelities and despised opinions. Jessica refused to see her at all until, in a great scene in the book, they met at the bedside of the dying Nancy in 1973. The price Diana paid went beyond estrangement from her family. She had to endure a harrowing three years’ internment in Holloway prison during World War II for supporting her husband’s activities and, on release, the permanent status of a social pariah. Till her death in Paris at 94 she was unrepentant. “Being hated means absolutely nothing to me, as you know,” she wrote to Deborah in 2001.

Thompson emphasizes that Diana could be kind, loyal and protective (and like her other sisters, a very good writer), but watching a Thames TV interview with her at the end of her life I felt an evil undercurrent to her hauteur. Why did she and Unity find the shimmer of totalitarian violence so attractive? Why were they dazzled by the glamour of authoritarianism — the jackboots, the rallies, Unity’s “darling storms,” as she appallingly called the Nazi storm troopers? Why were even their milder siblings — placid Pam, brother Tom, and their refined, aloof mother, Sydney — also fascist sympathizers, happy to visit Unity in Munich and socialize with Hitler? Why was Jessica drawn to — or blind to — Stalin’s nominally left-wing brand of murderous tyranny? Perhaps it was a fearful sense that the British ruling class was becoming irrelevant, that it represented a crumbling world order that war and depression had hollowed out. Perhaps they imagined that a strongman’s brute force and absolute certainty would protect them, along with the rest of humanity, from chaos and confusion. Thompson doesn’t entirely nail an explanation, perhaps because it’s impossible to know.

Through all this drama, the Mitfords’ rivalries were as intense as their loyalties. Thompson makes it clear that Diana is the still, chill touchstone for them all. The spiky, possessive Nancy was forever jealous when her own admirer Evelyn Waugh fell at Diana’s feet. After the war the Mosleys exiled themselves to Orsay in France. Nancy loyally visited Diana but never introduced her to her glittering circle of Paris friends. Competition with Diana also stoked Unity’s determination to outdo Diana’s fascism by following Hitler. A tinderbox dynamic played out through all their lives — Jessica, eloping with the radical Communist firebrand Romilly because Unity was a Nazi, Unity becoming a Nazi because Diana was a fascist. Nancy ricocheted back and forth with her alli­ances and feuds. When Diana was in Holloway prison she had no idea that Nancy had secretly testified against her, telling the British Foreign Office that Diana, who had given birth to a son seven weeks before, was as dangerous as Mosley himself. It was an act of self-righteous treachery that seems fueled more by Nancy’s long-suppressed jealousy and her sorrow at her disappointments in love than by her disapproval of Diana’s allegiance to Mosley’s repellent politics.