>>>The Untold Story of JFK’s Sister, Rosemary Kennedy, and Her Disastrous Lobotomy

More than seven decades ago, JFK’s troubled sister Rosemary was left disabled by a disastrous lobotomy ordered by her father. As Thursday marks the 100th anniversary of her birth, here’s a look at the heartbreaking details of a dynasty’s darkest secret.

“Darling Daddy,” 22-year-old Rosemary Kennedy wrote in a 1940 letter to her father Joseph P. Kennedy, then serving as U.S. Ambassador to Great Britain. “I am so fond of you. And I love you so very much.”

Just over a year later, that same young woman – affectionate, dutiful and always eager to please her father – was unable to form a sentence. In what would become a decades-long secret and a source of deep shame for the most famous dynasty in American history, Joe and Rose Kennedy’s intellectually disabled eldest daughter lost everything at age 23, when her father scheduled a catastrophic lobotomy that left her with the mental capacity of a toddler.

After the secret surgery, it would be another two decades – after Joe became incapacitated by a severe stroke in 1961 – before any of Rosemary’s eight siblings would learn the truth about their sister’s disappearance: she was living at a Catholic facilty for the mentally disabled in Jefferson, Wisconsin, hidden from public view.

Now, for the first time, two new books offer a devastating portrait of the lovely, troubled woman whose life was cruelly derailed by her father.

In Rosemary: The Hidden Kennedy Daughter, excerpted in this week’s PEOPLE, author Kate Larson depicts a vivacious daughter with a “perfect smile” who struggled to find her place in a family that prized achievement and success above all else. For the first time, Larson details the chilling events that deprived Rosemary’s brain of oxygen during her birth on September 13, 1918.

Larson writes that when Rose went into labor with Rosemary, her third child, the nurse who was caring for her was reluctant to deliver a baby without a physician on hand. Though the nurse had the necessary training, when the doctor’s arrival was delayed she demanded that Rose “hold her legs together tightly in the hope of delaying the baby’s birth.” When that failed, she resorted to “holding the baby’s head and forcing it back into the birth canal for two excruciating hours.”

As Rosemary grew into toddlerhood, Rose noticed she “was not like the others.” The family did their best to incorporate her into their daily lives, taking her sailing and making sure she was always asked to dance at parties. But as Rosemary got older, she began to have tantrums that sometimes turned violent. At the same time her voluptuous figure was attracting male attention, and Joe became concerned: an unwanted pregnancy in the family could damage his sons’ political future. “The family tried to protect her,” says Larson. “But the situation was a ticking time bomb.”

In November 1941, Joe scheduled Rosemary for a lobotomy, an experimental procedure meant to make mentally ill patients more docile. The surgery, writes Larson, involved drilling holes on both sides of Rosemary’s head, inserting a spatula into her cranium near the frontal lobes and turning and scraping. The surgery was botched and Rosemary emerged almost completely disabled.

After housing her in a psychiatric facility in upstate New York for seven years, Joe ordered his daughter sent to Saint Coletta and never saw her again. Her siblings didn’t see her for two decades.

In The Missing Kennedy, also excerpted in this week’s PEOPLE, author Elizabeth Koehler-Pentacoff shares memories of visiting Rosemary at Saint Coletta, where Koehler-Pentacoff’s aunt, Sister Paulus, was one of her caretakers for over thirty years. “Rosie was happy when she had visitors,” says Koehler-Pentacoff. “She loved parties and music and sweets. If we said we brought a box of candy, her eyes lit up. When people visited her, she was in heaven.”