Sisters Jackie O and Lee Radziwell feuded for years. But they always loved each other.

They were two of the most glamorous figures of their generation; their social lives among the world’s elite once the envy of women all over the world, and polished wardrobes a sartorial reference to this day.

But behind the shiny facade of socialite sisters Lee Radziwill and Jackie Kennedy Onassis was a bitter feud.

One so bitter, it became impossible to keep out of the spotlight, with public outbursts of jealousy dotting the life of the late Lee, who died in New York City on Saturday at the age of 85.

Lee, it seems, had always lived in her older sister’s shadow.

Now, as the world mourns her loss, the details of her complex relationship with Jackie - in particular their noted rivalry over the royals and aristocrats they married and bedded, are being unearthed.

Born Caroline Lee Bouvier in 1933, Lee grew up to rub shoulders with a myriad of artistic celebrities and fashion moguls.

Known for her immaculate sense of style, she was frequently featured on international best-dressed lists and on the cover of Vogue.

She had careers in acting, writing, and interior design, and was married three times - including to a Polish prince.

Lee's first husband was Michael Canfield, son of the president of the Harper & Brothers publishing house.

She later married Polish Prince Stanislas Radziwill, with whom she had two children.

During their relationship, she lived in London and insisted on being addressed as Princess Lee Radziwill, even though her husband took on the name of Mr Radziwill when he took British nationality in 1951.

Her third marriage was to Herbert Ross, director of the iconic films Footloose and Steel Magnolias.

While Lee was said to be close to her sister Jacqueline, who died in 1994, reports of a vicious rivalry were never-ending.

In an interview with the New York Times in 2013, Lee said she felt "lucky that there was so much more interest in my sister", but that "at times it was annoying, at times funny".

"Perhaps the most depressing part was that whatever I did, or tried to do, got disproportionate coverage purely because of Jackie being my sister."

In early 1961, at a swinging party at Lee's lavish London home, she had reportedly made her jealousy known to guests.

Jackie was about to become the most celebrated First Lady in U.S. history, while married to John F Kennedy, an aspect of Jackie's life which filled Lee with hostility towards her sister.

"How can anyone compete with that?" Lee had asked guests. "It’s all over for me now."

In a new book on the life of the Bouvier family, it's suggested that Lee's jealousy ran deeper, that she was the black sheep of the family while Jackie was the favoured daughter.

In Jackie, Janet & Lee, by Hollywood biographer J. Randy Taraborrelli, interviews with family members reveal their mother, Janet Bouvier Auchincloss, had passed down her own affinity for the high life, moulding her daughters' social-climbing image on her own.

It reveals that when they were young, Janet would take the sisters to tea at New York’s Plaza Hotel, occasions on which she would also impart some motherly advice.

"Do you know what the secret to happily-ever-after’ is?" she once asked them. "Money and power."

This, she said, was why the girls must always "marry up".

Apparently, this was a notion which shocked a young Lee, while Jackie, three years older, was perfectly accepting of the idea of a loveless marriage entered purely for money.

Janet was only 21 when she wed Jack Bouvier, the girls' father - a heavy-drinking Wall Street stockbroker and socialite who had several affairs before their divorce 12 years later.

Janet married a second time, to a wealthy investment banker, Hugh Auchincloss, with whom she had a daughter, also Janet, and son, James.

But while Janet had claimed she would encourage Jackie and Lee to always support each other, she and Jack Bouvier had forever displayed clear favouritism towards Jackie.

And when Jackie became the most famous woman in the world, Lee felt even further thrust into her shadow.

Lee was the rebel, while Jackie, most saw as the more intellectual and "together" Bouvier daughter.

In Taraborrelli's book, stories of Janet's hard-nosed parenting approach suggest the girls were somewhat afraid of disappointing their mother when it came to who they should marry.

In particular, Jackie, who had learnt to "play the game" with finesse based on her mother's advice, while Lee would continuously rebel and insist she'd rather marry for love.

When Jackie lost her virginity at 21 to John Marquand Jr, son of a celebrated writer, Janet was incensed. He was penniless, which warranted a slap across the face.

Janet instead steered her towards a British-educated stockbroker, John Husted. But when she discovered he was not as wealthy as she had thought initially, she had their engagement called off.

Jackie, who had learnt her lesson and was becoming a mirror image of her mother, simply slipped her engagement ring into Husted’s jacket pocket when she next saw him.

"She was ice cold. Like we never knew each other," Husted recalled.

"I never heard from her again."

On the other hand, when Janet tried to step in to Lee's relationship with Michael Canfield, a young publishing executive, she faced great difficulty.

Lee defied her mother, insisting she would marry for love. She was also determined to say her vows before her sister, who had then began dating a young U.S. Senator, Jack Kennedy.

Lee was 19 when they wed, but when Canfield couldn’t fund the expensive lifestyle she expected, she left him - not without Janet's intrusion, of course.

When the couple went to visit the family in the US, Janet ordered him to leave, informing her daughter: "He’s gone, Lee. I took care of it".

Canfield later claimed Lee had boasted to him of having sex with Jackie's husband, JFK, while the two couples were on holiday together. This wasn't the only time the rumours were heard.

Nini Vidal, a relative of Jackie and Lee, claimed she had witnessed the affair herself. She said Lee reportedly left her bedroom door open deliberately so Canfield could hear her and JFK having sex.

The tryst occurred while Lee was staying with the Kennedys after Jackie had given birth to daughter, Caroline.

After Canfield, she married Polish Prince Stanislas Radziwill, but reportedly "grew bored" of the marriage. A marriage that was riddled with affairs anyway.

She struck up a relationship Aristotle Onassis, one of the richest men in the world, and who was 27 years older than Lee.

‘Ari’ told friends he found Lee's complex relationship with her famous sister alluring. He once called her a ‘sad and lonely little creature’, desperate for attention.

Of course, he went on to wed Jackie, instantly drawn to the rather regal Bouvier sister when she came on a cruise with the couple.

"Ari is what I deserve" Lee had claimed.

But shortly after losing her baby son, Patrick, Jackie joined Lee and Onassis on a cruise in October 1963. This was the beginning of the sisters' infamous public falling out.

"So this is how kings live," Jackie had whispered to him coquettishly when he kissed her on the cheek.

On November 22, 1963, in the hours after JFK’s assassination, Onassis tried to call Jackie on a private number she had given him, while, of course, still in a relationship with Lee.

He flew to the White House to comfort Jackie. Reportedly, JFK’s blood was still fresh on her pink Chanel suit.

A year after the president’s death, they started an affair, however Jackie was still involved with other men.

Onassis wooed her with his wealth, gifting her with jewels every night as they dined on private islands, yachts and in European mansions, just as he had her sister years earlier.

But while Onassis seemed the ideal man for Janet's vision for her daughters, she drew a line, knowing how far he would divide her daughters.

She did her best to send him on his way, but her attempts were futile. He and Jackie married in 1968.

By the eighties, the sisters were seeing far less of each other.

When Janet died after a battle with Alzheimer's, the sisters grew even further apart. Jackie’s resentment increased when she discovered Janet had given Lee a large sum of money - said to be a ‘guilt’ payment for the way she had favoured Jackie throughout their childhood.

Jackie was diagnosed with cancer in 1994, and died in May that year, aged 64, at home in New York.

But despite their years of bickering over immensely wealthy men, and the power and money that came with them - the very goals their mother had set for them, Lee was at her bedside, holding her sister's hand as she rapidly declined.

"I love you so much. I always have, Jacks. I hope you know it," she told her dying sister.