Now, however, the story of that fight — and other new allegations — has been thrust into the spotlight with the publication of Ms. Trump’s memoir, a copy of which The New York Times obtained on Tuesday. The book, along with a number of court documents that have never been reported, sheds new light on a decades-long saga of greed, betrayal and internecine squabbles, laying out what Ms. Trump has described as her family’s legacy of darkness and dysfunction.

Her book, “Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man,” which is set to be released next week, has ended up in court itself: The Trump family has sought to stop its publication. Ms. Trump has countered that the secrecy provision that has kept her silent until now is unenforceable and based on financial fraud.

A memoir by President Trump’s niece, Mary L. Trump, sheds new light on decades of family infighting. Doug Mills/The New York Times

The book makes a number of allegations that Ms. Trump depicts as family secrets, among them a claim that a young Donald Trump paid someone to take his SAT, the standardized test used for college admissions. It also alleges that Mr. Trump’s sister, Maryanne Trump Barry, a former federal judge, considered him “a clown” who had “no principles” and that the Trump family left Fred Trump Jr. unattended at a hospital on the night that he died.

In her book, Ms. Trump seeks to explain how Donald Trump’s position in one of New York’s wealthiest and most infamous real-estate empires helped him acquire what Ms. Trump has referred to as “twisted behaviors” — attributes like seeing other people in “monetary terms” and practicing “cheating as a way of life.”

Ms. Trump, a clinical psychologist, calls her grandfather — the president’s father, Fred Trump Sr. — a “sociopath” who damaged his children. His father’s behavior, she concludes, led the president to adopt bullying and other aggressive behaviors to mask his own insecurities.

While several close associates of Mr. Trump have published exposés of him and his time in office, Mary Trump, who is 55 and lives on Long Island in New York, is the first member of the family to have broken ranks by writing a book.

Sarah Matthews, a White House spokeswoman, said Tuesday that the book was in Ms. Trump’s “own financial self-interest.” She said the president has described his relationship with his father in warm terms and called the allegation about the SAT “completely false.”

A lawyer for Mr. Trump’s family, Charles Harder, did not respond to an email seeking comment.

John Barrengos, one of Ms. Trump’s oldest friends, said that he believed the book was her response to a family that she feels tried to silence her and an attempt to shed light on her uncle, whose politics she strongly opposes.

“I think trying to tell the story as she sees it is a way of again claiming her voice not just in the construct of the family, but in the context of what our country is going through,” Mr. Barrengos said.

A turbulent family

The seeds of Ms. Trump’s alienation began before she was born, with her father’s relationship to his family, and continued through her childhood before bursting open when her grandfather died, according to her book and court documents, some of which remain under seal.

Ms. Trump and her younger brother, Fred Trump III, were the only children of Fred Trump Jr., the oldest sibling of Donald Trump, and Linda Clapp Trump, a onetime flight attendant who did not win her father-in-law’s approval. The couple had their son in 1962. Mary Trump was born three years later.

Fred Trump Jr. was not inclined to the family real-estate business, so Donald Trump stepped into the role of his father’s successor. The eldest Trump sibling became a pilot and struggled with alcoholism.

In her book, Ms. Trump writes that her uncle Donald watched her grandfather mock her father, learned from the ridicule to become Fred Sr.’s favorite son and joined in it. Donald Trump told his brother, referring to his career as a pilot: “Dad’s right about you: You’re nothing but a glorified bus driver.”

Donald Trump and his father, Fred, outside the Plaza Hotel in 1988. Ron Galella Collection, via Getty Images

For a child of one of New York’s most successful families, Ms. Trump had a turbulent upbringing. Her father was clashing with his own father and younger brother, she writes, drinking and smoking heavily. They lived in a drafty apartment in Highlander Hall, a Trump building in Queens, and at one point she was hospitalized with pneumonia.

Her father started to spiral downward. He had tried to buy a house but could not get a mortgage. “Our family was effectively trapped in that run-down apartment in Jamaica,” she wrote. “At 29 years old, my father was running out of things to lose.”

On one occasion, young Mary woke up to her father laughing while aiming a gun at her screaming mother’s face, she wrote in her book. By 1970, her mother told her father to leave, and he would never live with them again. They divorced in 1971. Fred Trump Jr. died of a heart attack in 1981 at age 42.

His children, who had already been given $400,000 each in trust by their grandfather, inherited a 20 percent stake their father had been granted in Trump apartment buildings in Brooklyn and Queens, several ground leases and other revenue-producing businesses.

Long after their father’s death, Mary Trump and her brother continued attending family events, including a Mike Tyson fight in Atlantic City with Donald Trump, their grandfather’s birthday party at Peter Luger Steak House, Ivanka Trump’s eighth birthday party and weddings, holidays and visits with their grandmother.

Still, they remained at the edges of the family. Fred Trump Sr. never liked Linda Trump, according to testimony in a battle over his will, and worried that any money left to his two grandchildren would end up in her hands.

“He had a tremendous dislike for their mother,” Donald Trump said of his father in a deposition obtained by The Times. “He felt the mother was the cause of Fred’s difficulty.”

Fred Trump Sr. also looked down on Mary Trump and her brother because of what he perceived as a poor work ethic fostered by inheriting their father’s money, according to testimony in the will dispute by John Walter, Donald Trump’s cousin.

“He knew what Fred III was doing,” Mr. Walter testified. “He knew what Mary was doing. He knew what their father had done before them.” Fred Trump III, Mr. Walter said, was “not working hard enough.”

Although Mr. Walter said that Mr. Trump Sr. did not expect Mary Trump, as a woman, to work in construction, he did not think either of the children was fulfilling their potential.

A fight over inheritance

When Fred Trump Sr.’s will was revised in 1991, he left $202,000 to each grandchild, including Mary Trump and Fred Trump III. The bulk of the Trump fortune would pass to his four living children. His other grandchildren stood to eventually inherit their parents’ portion. But Mary Trump and Fred Trump III — without their knowledge — were cut out of a 20 percent share of their grandfather’s estate that they might have received had their father lived.

“This is tantamount to disinheriting them,” an adviser told the Trump patriarch in a memo before the will was finalized. “You may wish to increase their participation in your estate to avoid ill will in the future.”

After Fred Trump Sr. died on June 25, 1999, Mary Trump and Fred Trump III learned that they had been cut out. Nine months later, they contested the will in court in New York, arguing that their grandfather had been suffering from dementia and that his children had manipulated him to influence the way the will was written.

A week after they went to court, a Trump family company cut off health insurance to Mary Trump, her mother, brother and her brother’s family, including Fred III’s 9-month-old son William, who had suffered from seizure disorders and would be diagnosed with cerebral palsy. Donald Trump acknowledged the termination of the insurance was related to the fight over his father’s will.

“When [Fred Jr.] sued us, we said, ‘Why should we give him medical coverage?’” he told The Daily News at the time. Mary Trump told the newspaper that by contesting the will she was fighting for their father to be recognized. “He existed, he lived, he was their oldest son. And William is my father’s grandson,” she said.

Litigation over the will and the health insurance became the vehicles for the Trumps to hurl insults and raise grievances that had hung in the air for years.

Fred Trump Jr. struggled with alcoholism. Louis Liotta/New York Post Archives, via Getty Images

In an affidavit in a lawsuit over the health insurance, Mary Trump said that at a meeting at the Drake Hotel, her uncle Robert tried to persuade her and her brother to accept the will’s terms, mentioning how much had been spent on William’s medical care. They interpreted the statement as a threat to terminate the insurance if they fought the will.

Robert Trump, in his own affidavit, called William’s 24-hour nurses “highly paid babysitters.”

Fred III said he was shocked that his family would trivialize his son’s medical care.

“My loving aunts and uncles, in an expression of their undying concern for William, were more than willing to jeopardize his care in order to punish me and my sister,” he said in his affidavit.

Those aunts and uncles had not visited William at a hospital a short cab ride from their Manhattan apartments, though in a restaurant Donald Trump “yelled across the tables that he had heard my child was sick,” Fred III later said.

‘Absentee grandchildren’

The fight over the will was equally bitter.

“They live like kings and queens,” Donald Trump said of his niece and nephew in his deposition. “This is not two people left out in the gutter.”

Maryanne Trump Barry, for her part, testified there was “no relationship” between Mary and Fred III and her father, calling them “absentee grandchildren,” even as she acknowledged that they had attended Christmas at her parents’ house and other family events.

“They often came and left very early,” she said. “On each time they came Freddy was never wearing a tie, which drove my father bananas, and Mary was in pants and a baggy sweater, which drove him bananas as well.”

Mary Trump, in response, gave her lawyer a long list of the events they had attended.

In her book, Ms. Trump accuses Robert Trump of telling her and her brother during the will battle that if they did not settle, the family would bankrupt one of the companies in which they had inherited a stake and saddle the two of them with the bill.

Ms. Barry and Robert Trump did not respond to requests for comment.

The Trumps settled their disputes in April 2001, court records show. As part of the deal, Mary and Fred III received an undisclosed cash settlement, and they agreed to turn over the 20 percent stake in Trump assets they had inherited from their father, including seven apartment complexes, ground leases and stakes in a public housing complex and in the company Robert Trump had purportedly threatened to bankrupt.

After The Times reported on the family’s questionable valuations of its real-estate assets in 2018, Mary Trump concluded that she and her brother were duped in the settlement, she has claimed in the run-up to publishing her book.

Even as the court fight over the will was starting to be resolved, Ms. Trump tried to establish her own life.

After working on a master’s degree in English at Columbia University, she switched directions and in 2001 started taking psychology courses at Adelphi University, not far from her home. In 2003, she earned a master’s degree, and by the end of the decade had finished her doctoral studies, writing a dissertation that examined the qualities that made people vulnerable to being stalked by their partners.

Around the same time, she entered into a romantic relationship. Ms. Trump and her partner raised a daughter before separating several years later.

When her uncle Donald announced that he was running for president in June 2015, Ms. Trump did not take it seriously, assuming, she wrote, that he “simply wanted the free publicity for his brand.” Throughout the campaign, which was marked by scandals like the release of the “Access Hollywood” tape, Ms. Trump did not speak out, fearing that her voice would not be heard and that her views would make no difference, she wrote in the book.

She stayed in touch with her aunt, Ms. Barry, whom she quotes as saying about the presidential race, “He’s a clown — this will never happen,” during one of their regular lunches in 2015. Ms. Barry was particularly baffled by support for her brother among evangelical Christians, according to the book.

On election night, however, Ms. Trump took to Twitter, writing, “Worst night of my life.” She also wrote: “We should be judged harshly,” adding, “I grieve for our country.”

Ms. Trump has grown apart from the brother with whom she had been aligned in the family conflict years ago. While she has chosen to speak out against the family, he has taken a different path, nurturing a relationship with their uncle. In a statement released through the Trump family last month, Mr. Trump III distanced himself from his sister’s book and said their legal settlement had been generous and his son well-provided for.

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Correction: July 8, 2020 An earlier version of this article misstated the birth order of Ms. Trump and her brother, Fred Trump III. She is the younger, not older. It also misidentified a member of the Trump family who sued over an estate. The quote referred to Fred Trump III, not Fred Trump Jr.

Alan Feuer covers courts and criminal justice for the Metro desk. He has written about mobsters, jails, police misconduct, wrongful convictions, government corruption and El Chapo, the jailed chief of the Sinaloa drug cartel. He joined The Times in 1999.

Michael Rothfeld is an investigative reporter on the Metro desk and co-author of the book “The Fixers.” He was part of a team at The Wall Street Journal that won the 2019 Pulitzer Prize for national reporting for stories about hush money deals made on behalf of Donald Trump and a federal investigation of the president's personal lawyer.

Maggie Haberman is a White House correspondent. She joined The Times in 2015 as a campaign correspondent and was part of a team that won a Pulitzer Prize in 2018 for reporting on President Trump’s advisers and their connections to Russia.

A version of this article appears in print on July 8, 2020, Section A, Page 1 of the New York edition with the headline: Secrets and Bad Blood Inside the Trump Family.