For almost 20 years Gore Vidal and Jackie Onassis have had little in common except their late stepfather, Hugh D. Auchincloss, their mutual dislike and their membership in what society writer Stephen Birmingham once called "the definitive family of American Society." Yet last month the two warring wings of the House of Auchincloss found themselves improbably united in a common cause: They were separately and discreetly co-hosting literary soirees in Manhattan and Washington for a novice author. Of course, she's not just any neophyte. She is Nina Gore Auchincloss Steers Straight, 44, Vidal's irrepressible half sister and Jackie's stepsister, whose satiric first novel, Ariabella: The First (Random House, $9.95), has touched off the busiest guessing game of the literary season.

At stake are not issues of artistic merit ("An interesting failure," sniffed Publishers' Weekly), but whether Nina was writing fiction or a Bouvier Bildungsroman. Press insiders gleefully twittered over parallels between the novel's story of three blue-blood sisters and Nina's own rearing in the Auchincloss household as the overshadowed stepsister of Jackie and Lee. The New York Times noted "characters who might be said to closely resemble her and those closest to her," and the Washington Post zeroed in on Hammersmith Farm—the Newport estate of the Auchinclosses—as the novel's setting.

The family itself is keeping a stiff upper lip. Jackie, an editor at Doubleday, deplores any comparisons of herself to the calculating and brainy Savannah of the novel. "There's autobiography in every novel," she says. "Nini [as the family calls her] can really write. You are just reading along, and suddenly you find yourself roaring with laughter." Nina simply dismisses notions that it is a roman à clef as "nonsense."

But others in the family disagree. "I don't think anyone wouldn't recognize the main characters—Jackie is Savannah, Lee is Melanie, my father is Uncle Melville and my mother is Aunt Miriam," says Nina's half brother Jamie Auchincloss, a 34-year-old photographer. "Nini," complains Jamie, who is the progeny of Nina's father, Hugh Auchincloss, and Jackie's mother, Janet Bouvier, "said some rather rough things about my father." (Perhaps revealingly, Nina's big stepsister Lee Radziwill was conspicuously absent from the Manhattan launch party.)

Nina's is actually the third novel to spring from an Auchincloss brow this spring. Vidal recently bowed with his 17th, a fifth-century B.C. epic, Creation, while cousin Louis Auchincloss, a prominent Wall Street lawyer, clocked in with his 18th, a tale from the court of Louis XIV, The Cat and the King. "Our family," says Vidal (envisioning the relationships at right), "is rather like a conglomerate. We've acquired the most elegant of predators, the best novelist of high society, the most beautiful nymphet around, and this notorious Attorney General. It's not exactly the family next door."

Indeed, "the lacy branches of the Auchincloss family tree spread across American Society's entire landscape," as Birmingham once grandly put it. The family patriarch, Hugh Auchincloss, was a Standard Oil heir whose three marriages yielded seven children and stepchildren and a mother lode of ambition, wealth, triumph and tragedy. Nina was born to Hugh's second wife, the darkly beautiful socialite Nina Vidal (Gore is her son from her previous marriage). "Our grandmother used to say about our mother, 'She is like an evil spirit when she comes into a room,' " reports Gore. Nina recalls that "I never saw my mother and father together. I was raised by lots of nannies until I went to live with my stepmother." Just as well, according to her half brother. "My mother," says Gore, "was an alcoholic when she had me. By the time Nina came along she was certifiable."

After her parents' divorce, Nina was boarding at an L.A. convent school, Marymount, when she received a letter from her mother. "She decided that I would be left in the West," says Nina, "and she and my little brother Tommy would go live in Washington." Distraught, Nina mailed the letter to her father, who was summering with his third wife, Janet, at Hammersmith Farm. "Janet said, 'That's not right! We'll take her in,' " remembers Nina. "What family life I have had I owe to her." (Nina's mother remarried once again and died in 1978.)

Nina formed a fast alliance with Jackie. "Nina," says Mrs. Onassis, "was just a marvelous thing that came along with our new life of a stepfather and another family. We liked the same things—horses and books." Both girls were given to pranks. "We once managed to get Lee in the pool," laughs Nina, "and leave her with this Airedale terrier, Woofty, knowing he would never let her out. Lee was a very sexy-looking girl at an early age, but not athletic. Jackie and I were."

Not surprisingly, growing up with those dazzling Eastern debutantes left its bruises on the neglected girl from the West. "I got all Jackie's and Lee's hand-me-downs," recalls Nina. "I always looked like a trendsetter in a slightly different size. The hem would be up or down or floating around."

Auchincloss was a passive father who expected Janet to rule the roost of seven children—two of hers, three of his and two of theirs. "When Nina had problems," says Vidal, "she would come to me. I was always around for her, and I was the only male in the family who was." Jackie—one of Vidal's favorite targets (he satirized her in his 1970 book Two Sisters)—shared with him that supportive feeling for Nina. "Both Gore and Jackie have huge senses of duty," says Nina. "When someone calls or goes down with a boom, one of them will show up."

After polishing at Miss Porter's and debuting at a ball for 600 at the Newport homestead in 1955, Nina briefly dated Teddy Kennedy, "my first and last beau," she says. Two years later, as a sophomore at Bryn Mawr (and a year before she was to come into a $225,000 inherited trust), Nina married lawyer-economist Newton I. Steers Jr., 20 years her senior and later a Republican congressman from Maryland. "It was all very 19th-century," explains Nina, referring to her courtship and marriage. Says a friend: "He rescued her from an intolerable life." After her marriage, Nina continued to commute three times a week from Washington to Bryn Mawr's Main Line Philadelphia campus for her degree in modern European history. She subsequently collected a master's in journalism from Columbia University in 1961 (while working part-time for columnist Charles Bartlett), followed three years later by an M.A. in history. "Nina used to be a historian," jokes friend Arthur Schlesinger Jr. of her changing interests.

She then worked as Washington correspondent for the Chattanooga Times from 1963 until 1971, while raising three sons. Her eldest boy, Ivan, 22, is now a construction worker in New York. "He has been on his own since 18," says Nina. "He wanted to find out what life is all about." Hugh, 18, and Burr, 15, are boarding at the exclusive Hotchkiss School in Connecticut.

It was at a birthday party for Ivan in 1961 that President Kennedy's daughter Caroline, then 3, fell off a flotation raft in Nina's swimming pool in Bethesda and was anxiously rescued. But the Steerses, both Republicans, were exiled from the Kennedy White House that year for having supported Richard Nixon. Nina was later chastised by stepmother Janet at a formal dinner at the Auchincloss Virginia estate, Merrywood, for her sin against the clan.

Seven years later Nina found her marriage to Steers unraveling. "It just ended," is all she will say now. Papers from their stormy divorce have been sealed in a Montgomery County (Md.) court since 1974. "Newton was not nice," says a friend of Nina's.

Before the divorce, Nina turned for comfort to one of her and Newt's closest friends, Michael Straight, whose own marriage was crumbling. He had been a former publisher of the New Republic, originally funded by his mother, Dorothy, the liberal daughter of robber baron William C. Whitney, a former Secretary of the Navy. "Nina is a loner who needs at rare moments to be supported," Straight wrote his mother in 1968. "There is a good deal in her that is self-defeating, even self-destructive." He now says his wife is "by definition a fighter. She has fought for everything she has ever gotten in life. And she regards the whole of life as a struggle." They wed in 1974, several months after her divorce from Steers. "It was sort of like marrying cousins," she says of their similar Establishment background. "We were very entwined."

Michael, 64, recently received some unexpected notice when his name surfaced in England as the source of information that helped break the Soviet spy network—fictionalized in Tinker, Tailor, Soldier, Spy. As a student Communist sympathizer at Cambridge, which he attended from 1934 to 1937, Straight had met future Soviet agents through an intellectual group called the Apostles. "In 1937," says Straight, "America was isolationist, and Russia was the only country which could stop Hitler." They asked Straight, who later wrote speeches for the Roosevelt Cabinet, to infiltrate Wall Street and submit economic appraisals. "I turned them down flatly," he says. "At age 21, I didn't know whom to go to, whom to talk to. I was frightened and appalled." Then in 1963 he gave evidence that later forced a confession from the man who had tried to recruit him, Sir Anthony Blunt.

Michael's main challenge now is keeping up with "the madwoman of Bethesda," as he affectionately calls Nina. Her novel published, she is in her second year of evening law school at American University and is simultaneously 2,000 pages into a biography of her maternal grandfather, blind Oklahoma Sen. Thomas Pryor Gore. "I want to finish up," she says. "Just get over the fence and down the pike."

One of the most independent and spirited couples in Washington, the Straights split their time between their five-bedroom home in Maryland and their seven-acre retreat on Martha's Vineyard. "This is a weird mix, the two of us," says Michael. "We lead our own lives and respect each other." Observes neighbor Jennifer Phillips: "You expect Nina to be very unconventional. But she is very conservative. There is this hard-core, very WASPy sensibility."

Looking back on the childhood she may or may not have captured in Ariabella, Nina thinks it a mixed blessing. "Everything was confusing," she says. "But all the splits and subdivisions have been colorful, and the people have been interesting. Those who had similar directions ended up with friendships that have the element of blood and family. Our family was like life," she muses, then adds a bit wistfully, "Life is rather relaxing in comparison."