Jackie's relationship with Onassis is thought to have tremendously hurt her sister, Lee Radziwill, who was reportedly romantically involved with Onassis in the early 60's, while Jackie was married to President John F. Kennedy.
Jackie kept her engagement to Onassis a secret – and didn't even tell her own sister. Onassis was the one to clue Radziwill in, and her reaction was swift.
Lee recently told Vanity Fair that Onassis "begged" her to come to the wedding.
Privately, Lee told trusted friend Truman Capote: "How could she do this to me!" according to Lee Radziwill biographer Diana DuBois.
To the outside world, however, she hid her inner turmoil. Before she boarded a plane from Paris to Athens for the wedding, she told reporters: "I am very happy to have been at the origin of this marriage, which will, I am certain, bring my sister the happiness she deserves," the Sydney Morning Herald reported in 1968.
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It was the final hit in a sisterhood that, yes, remained close throughout their lives, but was always fraught. From childhood on, the sisters had a close relationship, but there was always an undercurrent of competitiveness. Lee married first (to Michael Canfield, whom she divorced in 1959), but Jackie married better – to the very eligible JFK.
And though Lee's life in London, just blocks from Buckingham Palace, was incredibly glamorous – after divorcing Canfield, she married a prince, Stas Radziwill, becoming the princess of a Polish noble house – it was Jackie who captivated the world.
After Jackie's visit to France with JFK and the sisters' joint trip to India and Pakistan in 1962, she was on her way to being the world's most photographed woman. Lee, on the other hand, was increasingly hidden by her sister's larger-than-life presence.
But it was Jackie's marriage to Onassis that was the ultimate blow to their relationship. Years before, Lee herself had a friendship with Onassis. One that, at some point, she even considered ending her marriage for.
"Who didn't?" Lee responded when Vanity Fair asked if she ever considered marrying him.
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Vanity Fair speculates that it was her sister's growing star power that led Lee to seek out a new friendship – one with even more glamour than her royal husband. The friendship was forged with Greek shipping magnate Aristotle Onassis, who today, is perhaps best remembered as Mr. Jackie O.
At the time, Onassis was a sworn enemy of the Kennedy family – particularly Lee's brother-in-law, JFK. Robert Kennedy, too, wasn't a fan, and according to writer Evan Thomas, author of Robert Kennedy: His Life saw her friendship with Onassis as a "betrayal of the whole family."
Her husband's disdain for the man didn't stop Jackie from vacationing with Lee on Onassis's yacht, the Christina, after the premature death of her second son, Patrick in 1962. Thomas said that, really, the trip was also an opportunity for Jackie to attempt to convince Lee not to marry Onassis.
Little did Lee – and the Kennedys – know that Onassis and Lee's relationship wasn't going to be the problem. Five years after JFK's death, it was Jackie who tied the knot with the shipping magnate.