New Book Details Ted Kennedy's Relationship with Sister Rosemary, Hidden Away After Lobotomy

New York Times Co./Getty The Kennedy family, including Rosemary (left) and Ted (third from left) in 1938

From birth, the Kennedy children — five girls and four boys, including former President John F. Kennedy and the late Sens. Robert Kennedy and Ted Kennedy — faced a challenge to live up to the image put forth by their parents.

Most of the kids made it look easy. But Rosemary, the eldest daughter, was not like the others.

“Rosemary threatened the family’s desire for perfection," Neal Gabler writes in his new book, Catching The Wind: Edward Kennedy and the Liberal Hour.

The 736-page biography, published Oct. 27, offers a sweeping history of future Sen. Ted Kennedy, from his childhood through his decades-long political career. But it also sheds new light on Ted's relationship with his eldest sister Rosemary, who was intellectually disabled and underwent a disastrous lobotomy in her 20s.

To understand the Kennedy family's relationship with Rosemary, suggests Gabler, one must first understand how patriarch Joseph P. Kennedy Sr. viewed family in general.

"No one believed more fervently in family than Joseph Kennedy. No one was more invested in family than Joseph Kennedy," Gabler writes. "And of all the lessons he taught his children, there was none more important than this one, which he had learned from a lifetime of affronts: Family takes precedence over everything else."

"But if family was a refuge, a defense from the world, it was also, in Joe Kennedy’s mind, a weapon, an instrument to beat the world," according to Gabler. "And the Kennedys had to be trained, drilled, for it."

RELATED: The Untold Story of JFK's Sister, Rosemary Kennedy, and Her Disastrous Lobotomy

That competitive streak meant that Ted and Rosemary bonded over being the family underdogs.

Often teased for being chubby, Ted was the youngest of the nine children and shared a certain vulnerability with Rosemary — a "blot," Gabler writes — who could not go to traditional schools like her siblings and often stayed at home, bonding with the young Teddy during their time together.

Of the Kennedy's eldest daughter, Gabler writes: "Physically, Rosemary met the Kennedy standards, Rose’s standards. She was lovely. But as a child, she lagged behind her two older brothers’ benchmarks, was slower than they had been both physically and mentally, and when she was five and enrolled in kindergarten at the Edward Devotion School in Brookline, she was declared 'deficient' — what was then called 'mentally retarded.' "

Kate Larson's Rosemary: The Hidden Kennedy Daughter, published in 2015, detailed how Rosemary's birth, in which she was forced back into her mother's birth canal, may have contributed to her developmental issues.

Though Joe and her mother, Rose, sent Rosemary to a special boarding school in England, she still desperately sought her parents' approval, writing them letters about "how well she was doing, about how proud she hoped they would be of her," according to Gabler.

The couple was sympathetic, Gabler writes, but they soon realized their daughter couldn't live up to the high standards they had set for their children.

"In a family of competition, Rosemary wanted desperately to be her siblings’ equal ... She could never, however, match her siblings. Both Joe and Rose were deeply sensitive to her situation and to theirs," Gabler writes. "They understood that the Kennedys’ desire for perfection threatened Rosemary; they understood, too, that Rosemary threatened the Kennedys’ desire for perfection."

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Upon her return to the states and now 23 years old — and attracting male attention (which led her father to fear an unwanted pregnancy) — Rosemary became increasingly difficult for the family to manage, and Joe sought out medical options.

“The family tried to protect her,” Larson previously told PEOPLE. “But the situation was a ticking time bomb.”

After a psychiatric evaluation didn't prove helpful, Joe decided to turn to an experimental treatment, with consulting his wife, Rose.

The treatment — a prefrontal lobotomy — promised to tame the bouts of violence to which Rosemary was prone and "render her more docile," Gabler writes.

But the botched procedure did that and more, destroying Rosemary's remaining mental capacity. Many in her immediate family wouldn't learn what had really happened to her for 20 years.

"For her own good again, but also clearly for their own, Joe had [Rosemary] institutionalized afterward, in a psychiatric hospital in the upper Hudson Valley called Craig House," Gabler writes. "She was gone to them now."

Joe, who became incapacitated by a severe stroke in 1961, never saw Rosemary again.

After Craig House, Rosemary lived the rest of her life in a Catholic institution in Jefferson, Wisconsin, hidden from public view, where she was cared for by nuns until her death in January 2005 at age 86.

RELATED: Unseen Letters from JFK’s Sister Rosemary, Who Underwent a Lobotomy, Reveal Depth of Her 'Loss'

Out of sight, the forgotten Kennedy daughter became something of a figment for her family — her life a hairline fracture in the image they had worked so hard to maintain.

"The Kennedys had been constructed as an organism, in which each part worked in conjunction with the others to create the Kennedy solidarity and advance the Kennedy brand— all except for Rosemary, who was exiled precisely because she had no function save as a reminder of God’s will," Gabler writes. "And the Kennedys had been constructed as a movie, a beautiful image of beautiful people who were smart, ambitious, productive, fun, and happy—perfect."

To her brother Ted, Rosemary had always been an enigma: a "kind of ghost," in Gabler's words.

According to his biography, Ted once told an interviewer his sister was "a presence," one who was “almost gentler and tenderer and more loving even than other brothers and sisters.”

While "she seemed to have little influence" on his brothers, Rosemary’s life left a lasting impression on Ted, who found his sister remarkable, despite his family's attempts to hide her disability, according to Gabler.

He writes: "Ted often cited her as one of the most important influences in his life both personally ('Rosemary enriched the humanity of all of us,' he would write in his memoir) and politically, listing her alongside Honey Fitz [his grandfather] and his father and his brothers."