Jackie Kennedy's younger sister Lee Radziwill has died age 85 at her home in New York City.
Along with Jackie, the slender brunette style-icon was considered to be one half of the world's most famous sisters during the 1960s.
She had previously been said to be battling an undisclosed age-related disease, but a source told WWD that over the past week she had been in 'good health', present, and as 'elegant' as ever.
Her cause of death is yet to be confirmed.
Jackie Kennedy's sister Lee Radziwill (pictured left in 1967, right in 2016) has died age 85 at her home in New York City
Born Caroline Lee Bouvier in New York in 1933 to New York Stockbroker and socialite John Vernou Bouvier III, Radziwill came from a famous family but established herself as a noteworthy figure in her own right.
She boasted an exclusive list of close friends, from Andy Warhol to Truman Capote, and worked briefly as an actress before turning her hand to interior design and public relations.
For a time, the social doyenne was also an American Princess. After her first marriage to Michael Canfield collapsed, she married Prince Stanisław Albrecht Radziwill, a Polish aristocrat, in 1959.
Earning her the name 'Her Serene Royal Highness', the pair had two children together, before they divorced in 1974.
She married a third time to film director Herbert Ross in 1988. Their union lasted 13 years and they split in 2001, just months before his death.
Lee married Polish aristocrat Prince Stanisław Albrecht (right) in the 1960s, and assumed the title 'Her Serene Royal Highness'. (pictured: Lee with her husband Stanislaus after her first acting role in "Philadelphia Story", onstage in Chicago in 1967)
Lee Radziwill married three times. she split from her third husband, Herb Ross (pictured right at their wedding reception) in 2001
But there was one everlasting relationship that stood above the rest for Radziwill - her bond with her sister, Jackie.
Jackie and Lee, were two of the most glamorous women of their generation, but their closeness came with a fierce rivalry.
Described by Capote as ‘American geishas’ who existed only to captivate the world’s richest and most powerful men, the duo were said to be embroiled in a life-long battle to one-up each other.
From a young age the pair pitted themselves against one another, with Lee regarded as the 'pretty one', and Jackie the 'smart one'.
From a young age, Jackie (right) and Lee (left) were said to have been competitive with one another. Jackie was considered to be the 'smart one' of the duo, and Lee the 'pretty one'
The First Lady of the land, Mrs. Jacqueline Kennedy, and her sister, Princess Lee Radziwill (right) were considered the world's most famous sisters during the 1960s
The competition reach a tumultuous crescendo in 1961, when Jackie became the First Lady after John F Kennedy's election as president (pictured: First Lady Jacqueline Kennedy stands with Lee and her daughter Anna on the walkway outside the Oval Office, 1963)
'The competition went on throughout their lives: ‘Who’s going to be more popular? Who will marry first?'' said Randy Taraborrelli, the author of 'Jackie, Janet and Lee', to People.
In her book 'Happy Times', Lee remarked: 'One always looks up to older siblings for guidance. One tries to emulate them and follow their achievements. That’s what I did with my sister.'
But the contention came to a shattering crescendo when Jackie became First Lady in 1961.
'How can I compete with that?' Radziwill reportedly said during a function at Buckingham Palace. 'It's all over for me now.'
In her book 'Happy Times', Lee says she spent most of her childhood looking up to her big sister Jackie. They shared many ups and downs over the years
In the years that followed the pair are said to have shared a turbulent relationship, with several highs and many well documented lows.
Radziwill was a frequent visitor to the White House during her brother-in-law's short-lived presidency.
Canfield, her first husband, would later claim Lee had boasted to him of having sex with John F Kennedy.
Lee was staying with the Kennedys after Jackie had given birth to daughter, Caroline. She reportedly left her bedroom door open deliberately so Canfield could hear her and JFK making love.
The claims, however, have never been proven.
After JFK's assassination, Radziwill left a note on Jackie's pillow that read: 'Good night my darling Jacks — the bravest and noblest of all. L.'
After JFK's assassination, Lee left a note on Jackie's pillow that read: 'Good night my darling Jacks — the bravest and noblest of all. L.' (pictured: Lee and her son Anthony accompany Jackie and her two children in London, 1965)
Former first lady Jacqueline Kennedy and her sister, Princess Lee Radziwill, pray in St. Patrick's Cathedral in New York City after the assassination of Senator Robert F. Kennedy
As Jackie lay dying of cancer in 1994, Radziwill clambered to be at her sister's side and told her 'I love you so much. I always have, Jacks. I hope you know it.'
But in one final tension-fueled twist in their relationship, Jackie omitted her sister entirely from the will of her $150 million estate, citing years of hand-outs as the reason.
Her children were given half-a-million dollars each.
'She was the height of elegance, a true icon — an incredibly attractive woman who always adapted to whatever the circumstance. She was never overdressed or underdressed,' said Mathilde Favier, of Christian Dior.
'She was incredibly elegant and inspiring, and always ahead of her time because she was a fashion visionary. It’s very sad. It’s the end of an era.'
Before her retirement, Radziwill served for many years as a Public Relations executive for Giorgio Armani.
She was a stalwart attendee of New York Fashion Week, and regularly attended other fashion events around the world right up until her death.
Lee was considered to be a style icon throughout her life. Her passion turned into a career when she worked as a PR executive for Giorgio Armani (pictured in 1976)
She was a stalwart attendee of Fashion Week in New York, regularly appearing in Paris, Milan and London too (pictured: Lee attends the Giambattista Valli show as part of the Paris Fashion Week Womenswear Fall/Winter 2016/2017 on March 7, 2016)
'Regrets? I think everyone has regrets, and people who say they haven’t are either liars . . . or narcissists' Radziwill said to the New York Times in 2013.
'There have been many things in my life to have regrets about, in the sense I wish I could have changed them, or somehow made them not happen,' she continued.
'What I don’t have is envy. I’m perfectly content at this time of my life. I’ve done so many fascinating things and the greatest joy is that I continue to do interesting things and meet fascinating people.
Radziwill's daughter-in-law, Caroline, currently stars as a cast member on the Real Housewives of New York.
The 85-year-old died on Friday. She is survived by her daughter Anna Christina Radziwill.
Her son Anthony died of cancer in 1999, just one month after her nephew, John Kennedy Jr, died in a plane crash.
'When I was young, I used to think that everyone should die at 70...but my closest friends like Rudolf and Andy [Warhol] and to an extent Capote, let alone my close family...didn't even reach that age,' Radziwill said in the same New York Times interview.
'There is something to be said for being older, and memories.'