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Rosemary Kennedy, oldest sister of John F. Kennedy, would have turned 100 on Thursday — and new unseen letters shed light on the sibling, who was born with a disability and whose life was shrouded in mystery.

The letters, unveiled by People magazine, date back to 1938.

Back when her father, Joseph Kennedy, was ambassador to Great Britain, 20-year-old Rosemary traveled to Ireland and England for three weeks in the care of a chaperone, Dorothy Smyth, a young Irish woman who was hired by the family to look after her.

After leaving Ireland, Rosemary wrote several letters to her new friend, detailing her travels in Europe. Smyth’s family kept the letters secret until she died in the mid-1960s.

“The letters were part of our family history,” Smyth’s nephew Michael Fisher told the magazine.

The 66-year-old said his family gave the letters to Rosemary’s surviving sister, Jean Kennedy Smith, when she was the ambassador of Ireland. They had hoped Jean would display them at the JFK Library in Boston.

“My mother decided they should be returned to the Kennedy family,” said Fisher. “We got a formal and brief acknowledgment that the family had received them.”

Rosemary died in 2005 at age 86. The New York Times reported Joseph was worried that Rosemary’s illness, which may have stemmed from brain damage at birth, would lead her into situations that would damage the family’s reputation. He arranged for her to have a lobotomy at age 23.

Rosemary’s younger sister, Eunice Kennedy Shriver, recalled that doctors had told their concerned patriarch that the procedure, in which the frontal lobes of the brain are scraped away, would help Rosemary.

However, the operation reportedly reduced Rosemary to an infant-like state. The Kennedy family rarely discussed her publicly.

Rosemary spent the rest of her life receiving round-the-clock care from nuns at a Wisconsin institution, where she reportedly mumbled words and sat for hours staring at walls.

Fisher said the letters show Rosemary was filled with hope before undergoing the botched procedure that would forever change her life.

“There is a childhood innocence about the letters,” he explained to People. “When you read the letters or look at the pictures of her going to a ball in London in a formal dress when she was introduced to society and then you reflect on her lobotomy, that is the story of Rosemary. Everything faded after the lobotomy and she wound in a home for all those years, away from the public.”

The magazine revealed that in one letter, Rosemary vividly described a trip to Cannes, France and the excitement of “changing into three planes, all by myself.” When she landed, Rosemary wrote, “As I was going off the plane they wanted to take my picture. ‘Miss Kennedy, please[!]'”

Rosemary’s biographer Kate Larson also told People magazine the letters revealed the heartbreaking reality of Rosemary eagerly seeking a friendship from her hired companions before she underwent her lobotomy.

“The letters are important because they reflect Rosemary as much younger, intellectually, than her 20 years and that she had a full life and her family included her,” said Larson. “They were written before Rosemary’s lobotomy and they reveal the loss more acutely. To see the childish scrawl and her gratitude [to Dorothy] just makes the loss all the more real.

Getty Maria Riva

Actress Maria Riva, daughter of Marlene Dietrich. (Getty)

“For Rosemary, many of these hired companions were substitute girlfriends. She often became emotionally attached to them, signing her letters ‘Best Love from your darling Sweetheart,’ and ‘Lots of Love and Kisses.’”

In a letter written from the south of France, where Rosemary and her siblings joined their mother, Rose Kennedy, in 1938, she described encountering Marlene Dietrich’s daughter, Maria Riva, who would go on to become an actress also.

Marlene Dietrich [‘s] daughter is down here,” wrote Rosemary. “And very attractive.”

“Dietrich would begin an open affair with Rosemary’s father, Joe, that summer,” alleged Larson. “Maria, seven years younger than Rosemary, spent the days at the resort as Rosemary’s friend.”

Larson, who will be speaking at the John F. Kennedy National Historical Site in honor of Rosemary’s birthday, hopes the letters will reveal a new side to a woman who inspired the Special Olympics.

“People with disabilities have voices too and they deserve to be heard,” said Larson. “And Rosemary needs to be heard and to remind the public about what happened to her, and the way the world was, not so long ago.

“There is still stigma, especially with mental health issues and mental illness. There are millions of people suffering and their families are suffering. We need to talk about it and make changes in the world so people can get the resources they need to treat mental illness and live full lives. We still have a long way to go. Rosemary and her story is a way to move the needle forward.”