Lee Radziwill, the socialite and younger sister of former US first lady Jackie Kennedy, has died at the age of 85.

Anna Christina Radziwill said her mother died on Friday of natural causes.

Ms Radziwill shared her older sister’s affinity for fashion and globe-trotting, as well as her dark, wide-set eyes and high cheekbones.

They were confidantes as young women, and Ms Radziwill was a frequent guest at the White House during President John F Kennedy’s administration.

She was with the president when he made a trip to London in 1961 and Mr Kennedy was godfather to Ms Radziwill’s daughter, Anna Christina.

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Socialite Lee Radziwill was the younger sister of Jackie Kennedy (Evan Agostini/AP)

The Kennedys and Radziwills spent Christmases together in Palm Beach, Florida, and the sisters travelled to India and Pakistan.

Ms Radziwill helped select the wardrobe for what became one of Jackie’s signature moments — her trip to Paris with her husband in 1961.

“She had to travel a lot and liked to have me with her,” Ms Radziwill wrote in Happy Times, a memoir published in 2001, seven years after her sister’s death. “Apart from mutual affection, I think our strongest bond was a shared sense of humour.”

But tensions emerged after Mr Kennedy’s assassination in 1963.

Ms Radziwill had worried that her brother-in-law’s ascent would overshadow her and told Gloria Steinem for a McCall’s magazine interview that her life during the JFK years was “empty, cold, and not true”.

In 1968, Jackie wed the Greek billionaire Aristotle Onassis, whom Ms Radziwill had once thought of marrying, only to have her sister urge her not to.

Ms Radziwill’s life apart from her sister was eventful enough. She married a prince, Stanislas Radziwill of Poland, and had two children, Anthony and Anna Christina.

There were friendships with Ms Steinem, Rudolf Nureyev, Andy Warhol and Truman Capote, whom she joined for a 1972 Rolling Stones tour.

“I can see how people found him (Mick Jagger) sexy,” she told interviewer Sofia Coppola for a 2013 New York Times story. “But I found him a little repulsive.”

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Jacqueline Kennedy is followed by her sister, Lee Radziwill, during a visit to London (PA)

She began work on a film with collage artist and photographer Peter Beard about her childhood in East Hampton, New York. But after a few creative evolutions, and the introduction of filmmakers Albert and David Maysles, it became Grey Gardens, the classic documentary about her eccentric aunt and cousin.

Edith Bouvier Beale and her eponymous daughter were immortalised in the 1975 release, later a Broadway musical and Emmy-winning HBO movie.

Ms Radziwill also made forays into interior decorating and fashion event planning. A brief, unhappy acting career in the 1960s proved she was best at playing Lee Radziwill, with critics panning her work in a stage production of The Philadelphia Story and a TV adaptation of the Otto Preminger film Laura.

It seemed at times that tragedy followed tragedy. In the aftermath of the death of her beloved nephew, John F Kennedy Jr on July 16 1999, her own son died of cancer less than a month later, on August 10, at age 40.

Her 10-year marriage to Herbert Ross, her third husband, ended that year, as well. The choreographer and award-winning director of Funny Girl and Steel Magnolias died in 2001.

Her first marriage was to Michael Canfield in 1953. The marriage ended in divorce and was later annulled. Mr Canfield died in 1969.

She married Mr Radziwill, a descendent of Polish royalty who became an English subject, in 1959. They divorced in 1974 and he died nearly two years later.