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

Jacqueline Kennedy Onassis and her younger sister Lee Radziwill were two of the most famous sisters in the world — and also two of the most private.

Lee’s death on February 15 at age 85 marks the end of an era — and renews curiosity in their complicated relationship.

“It was never sort of black and white. There were always shades of grey, and when you try to paint that relationship with a wide brushstroke it never works because there were times when they were very close,” Jackie, Janet and Lee author J. Randy Taraborrelli tells PEOPLE.

“Then, on the other hand, it sounds pejorative, but it’s definitely true that Jackie could have set Lee up in a great way financially, and she never did,” Taraborrelli adds, referencing Lee’s financial difficulties later in life. “So when Lee needed money for this and for that, Jackie would give it to her or lend it to her, but it fell short of the huge gift that sort of would have set her up for life.”

Continuing, he says, “Jackie never gave Lee the kind of financial freedom that would have set her up for life, and that’s sort of a function of their relationship. She left her nothing in her will.“

Although Jackie granted Lee’s children, Anthony and Tina, each $500,000, Jackie, who died in May 1994, two years after being diagnosed with non-Hodgkins lymphoma, wrote in her will that she made no provisions for her younger sister, “For whom I have great affection because I have already done so during my lifetime.”

Speaking of Jackie’s decision, Taraborrelli previously told PEOPLE that she had to have been aware of what she was doing to “make that decision and make it so publicly.”

“Of all people Jackie knew that anything she put in writing was going to end up in the public domain eventually,” he shared.

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

Speaking about the two sisters, Taraborelli tells PEOPLE they were never able to “just sit down and have a really good talk and really be vulnerable with each other and really put their feelings out there as sisters.”

“That’s not the way that they were raised, and they just were never able to do that. I just kept waiting as I was writing the book for that moment, and the closest that they came to it was right before Jackie married Onassis, when she told Lee, ‘I need this,’ and Lee said to her, ‘Well, then you should have it,’” he added.

The author’s book, which came out last year, revealed new details of Lee’s five-year on-and-off relationship with shipping tycoon Aristotle Onassis — years before he married her sister.

Lee began seeing Onassis while she was married to Prince Stanislaw Radziwill, with whom she had two kids: Anthony and Christina.

Although the sisters had always been competitive with each other, things got even more complicated after Lee introduced her sister to Onassis, before the dual assassinations of John F. Kennedy and his brother Robert Kennedy.

Although Lee’s relationship with Onassis continued at this time, “as much as she wanted to be with him, she gave him up for Jackie,” Taraborrelli previously told PEOPLE. “She realized she would not be able to live with herself if something happened to Jackie and the kids because she had not allowed Onassis to be their protector.”

RELATED VIDEO: Jackie Kennedy Knew About JFK’s Affairs and Considered Divorcing Him: ‘She Wasn’t Naïve’

Taraborrelli went on to tell PEOPLE that the way the sisters handled their relationships with Onassis was indicative of why they didn’t enjoy a more intimate relationship throughout their lives.

“Everybody knew that Jackie and Onassis were seeing one another, and certainly Lee knew about it long before Jackie married Onassis. But they didn’t have that talk. They didn’t have that talk that sisters ordinarily would have,” he shares. “And then they could have had the big blowup and the big explosion and screaming and whatever it takes to get past it, and then they would have had that in their history. And then that would have brought them closer together… That’s the kind of intimacy that was missing from their relationship.”

RELATED: Carole Radziwill Mourns Death of Mother-in-Law Lee, Jackie Kennedy’s Sister: ‘She Was Loved’

However, for all the differences, the two sisters did share qualities in common.

“One thing about the sisters is that they did everything they could to go on with their lives,” he shares, adding that both Jackie, Lee and their mother Janet Auchincloss would make sacrifices in order to ensure their own wellbeing.

“They were survivors,” he adds. “They were never victims.”