Lee Radziwill Was 'Obsessively Jealous' of Her Sister Jackie Kennedy, Says Author of Book That Inspired “Feud”

"When Lee and Truman [Capote] had lunch the first time, there she is talking about her obsession with Jackie," author Laurence Leamer tells PEOPLE

Jacqueline Kennedy Onassis and Lee Radziwill's sibling rivalry has made headlines for years, and details of the sisters' dynamic continue to garner attention.

As a connoisseur and confidante of the world’s most beautiful, elegant and moneyed women, novelist Truman Capote always had a reverence for Radziwill, according to Laurence Leamer, author of Capote’s Women, a book that inspired the current FX series Feud: Capote vs. the Swans, about Capote’s adoration for gorgeous women he called his "swans."

In the beginning, the sisters and Capote were all friends, but that didn't last long, Leamer tells PEOPLE.

“Truman took [Radziwill's] side against her sister and she was absurdly jealous of Jackie,” he says. ”He was very, very close to [Kennedy Onassis] until he wasn’t.”

Related: Lee Radziwill in Her Own Words — From Her Relationship with Jackie O. to Her Love for Extravagance

When the competition between the sisters grew larger than their closeness — around the time Kennedy Onassis's first husband, John F. Kennedy, was elected president, imparting Kennedy Onassis with the title of First Lady — Leamer says the conflict became worse than ever.

“Lee was obsessively jealous of her,” Leamer notes. “She couldn’t get over it, and that was her first conversation with Truman. And it’s a very American thing: you meet somebody and tell them the most intimate details of your life, and when Lee and Truman had lunch the first time, there she is talking about her obsession with Jackie."

Related: Jackie Kennedy Onassis Left Sister Lee Radziwill Out of Her Will and Never 'Set Her Up for Life'

Back in 1975 and 1976, chapters of Capote's work-in-progress, Answered Prayers, were published in Esquire. His writing revealed Radziwill and Kennedy Onassis' most salacious secrets, which resulted in Capote's excommunication from their social circle.

"She hardly knows this guy and what is he doing? He goes out the door, he tells everybody,” Leamer says, later adding, "He talks about how Jackie looked more like a man, which is a pretty insulting thing to say about the president’s widow and how incredible looking Lee was."

Although the siblings' feud over what Leamer says was money, men (Radziwill dated Aristotle Onassis before he married Kennedy Onassis in 1968) and status was made public, but the sisters were there for each other behind closed doors when times got tough.

When JFK was assassinated in 1963, Radziwill, who died in 2019, flew to Washington D.C. from her London home to comfort her sister. When asked about their rivalry in 1976, Radziwill told PEOPLE, “It’s just the most ludicrous talk in the world. We’re exceptionally close and always have been.”

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Still, that rivalry always seemed to linger. When Kennedy Onassis died in 1994, her will revealed she would not be leaving any money to her younger sister.

“I’ve made no provision in this, my will, for my sister, Lee B. Radziwill, for whom I have great affection because I have already done so during my lifetime,” it read.

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