It was a rare admission when President Trump told The Washington Post last year that he regretted the way he treated his older brother, Fred Jr., who died of alcoholism. But he insisted that a subsequent financial feud with his late brother’s children had been settled amicably, saying, “We all get along.”

Now, however, Trump’s niece — the daughter of Fred Jr. — has written a book slated to be published in July that could explode the image of a unified Trump family.

In a description of the book posted on Amazon late Monday night, Mary Trump, a clinical psychologist, is said to describe “a nightmare of traumas, destructive relationships, and a tragic combination of neglect and abuse. She explains how specific events and general family patterns created the damaged man who currently occupies the Oval Office, including the strange and harmful relationship between Fred Trump and his two oldest sons, Fred Jr. and Donald.”

The Fix’s Natalie Jennings analyzes what former Vice President Joe Biden meant when he said President Trump would try to steal the 2020 election. (The Washington Post)

Mary Trump plans to reveal the feuds that have long simmered within the family but until now have been largely masked by nondisclosure agreements and the limited public visibility of the president’s siblings, the Daily Beast reported Sunday.

The book will include Mary’s account of her conversations with Donald Trump’s sister, former U.S. appellate judge Maryanne Trump Barry, who does not speak flatteringly about her brother, according to the Daily Beast.

Mary Trump, who declined an interview request last year to speak to The Washington Post for the story about her father’s death, did not respond Monday to a request for comment.

A spokeswoman for the publisher, Simon & Schuster, confirmed that the book, called “Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man,” will be published July 28, declining to comment further.

Barry, reached by phone Monday, declined to comment. The White House did not respond to a request for comment.

Mary Trump’s book, if it is as critical as has been reported, would mark a rare departure among the president’s three living siblings and extended family members, who have largely refused to comment about him and have stayed out of public view. Its publication threatens to put the Trump family’s internal tensions on prominent display months before the November election.

“They might be polite, but Donald’s personality is one of dominance, and that includes his family,” said Jack O’Donnell, a former Trump casino executive. “That is no fun for anyone.”

Some of the friction appears to stem from a battle over the family fortune that erupted after the death of Fred Trump Jr.

The future president’s older brother, Fred Jr., went to TWA’s training facility in Kansas City, Mo., and flew briefly as a secondary pilot. He died of alcoholism in 1981 at 42 years old. His children, Fred III and Mary, thought they would one day receive what they believed was their father’s fair share of the family wealth.

But 18 years later, when Fred Sr. died, Donald Trump and his siblings fought to keep most of the money for themselves, according to court records.

Fred III and Mary sued other members of the Trump family in 2000, alleging that their relatives had persuaded Donald Trump’s father to change the will.

Donald Trump responded harshly: He cut off family company payments that had been used to care for Fred III’s son, William, who had cerebral palsy.

Trump told the New York Daily News at the time that “when [Fred III] sued us, we said, ‘Why should we give him medical coverage.’ ”

Fred III said in court documents that “my aunt and uncles thought nothing about taking away my critically ill son’s coverage in an attempt to browbeat me into abandoning my claim in the probate contest.”

“You have to be tough in this family,” Fred III told the Daily News at the time. “I guess I have what my father didn’t have. I will stick to my guns. I just think it was wrong. These are not warm and fuzzy people. They never even came to see William in the hospital. Our family puts the ‘fun’ in dysfunctional.”

Fred III, who could not be reached for comment Monday, has not publicly spoken about the matter since the case was settled confidentially.

Mary told the Daily News at the time that she was incensed by the way the family treated both her family and her brother.

“Given this family, it would be utterly naive to say it has nothing to do with money,” Mary Trump said. “But for both me and my brother, it has much more to do with that our father be recognized. He existed, he lived, he was their oldest son. And William is my father’s grandson. He is as much a part of that family as anybody else. He desperately needs extra care.”

According to the Daily Beast, Mary Trump was a primary source for a 2018 investigation by the New York Times into how Donald Trump received hundreds of millions of dollars from the family real estate company.

The New York Times declined to comment.

President Trump has tried to gloss over the disputes within his family. Asked by The Post last year about the feud with his nephew’s family, he minimized the cost of caring for William Trump’s cerebral palsy. “One child was having a difficult time,” Trump said. “It was an unfortunate thing. It worked out well, and we all get along.”

In discussing his brother’s death with The Post, Trump did acknowledge that he regretted his constant criticism of Fred Jr. for seeking to be a TWA pilot instead of going into the family business. He recalled telling his older brother, “You’re wasting your time.” Their father, Fred Sr., berated Fred Jr. for wanting to be nothing more than “a chauffeur in the sky.”

“I do regret having put pressure on him,” Trump said. Running the family business “was just something he was never going to want” to do. “It was just not his thing. . . . I think the mistake that we made was we assumed that everybody would like it.”

Years earlier, Fred Trump Jr. had helped Donald when he was seeking to transfer from Fordham University to the University of Pennsylvania’s business school, Wharton. Fred Jr. was close friends with a Penn admissions officer, James Nolan, who told The Post last year that he interviewed Donald Trump for entry.

Contrary to Donald Trump’s assertion that Penn was one of the nation’s hardest schools to enter, showing that he was a “genius,” Nolan said more than half of applicants were admitted and that it was “not very difficult.”

During the inheritance fight, Donald Trump relied especially on his younger brother, Robert, who served as a spokesman for the siblings in court documents. In fighting against the effort by Fred III to gain support for his son with cerebral palsy, Robert Trump wrote that the family had provided financial support “out of the goodness of our hearts” and had given Fred III $200,000 annually even though they didn’t have to lift “a finger” for the money.

While Robert Trump acted as Donald Trump’s surrogate in the court case, he has had his own clashes with his brother, according to O’Donnell, a former Atlantic City casino executive who worked with both men. In his memoir, “Trumped!,” O’Donnell wrote that Donald Trump hired Robert to help run his casinos, which were then floundering.

When Donald Trump complained during a meeting with his casino executives that “We’re going to lose a fortune,” Robert responded: “Donald, you know there’s just no way to predict these things,” according to O’Donnell.

Trump was furious with his brother, saying, “I’m sure as hell not going to listen to you in this situation. I listened to you and you got me into this,” according to O’Donnell. Robert Trump said, “I’m getting out of here. I don’t need this,” O’Donnell wrote.

Donald Trump, in an interview for The Post’s biography, “Trump Revealed,” said his brother “never quit” and did a “really good job.”

O’Donnell said in an interview Monday that Donald Trump promotes the “image of a strong loyal family,” including with his parents, children and siblings. Although O’Donnell said Robert gets along with Donald, he thinks the browbeating at the Taj Mahal casino “changed forever” their relationship.

Robert Trump, who could not be reached for comment, does not appear to have ever spoken publicly about what happened in Atlantic City. He told the New York Post in 2016 that “I support Donald one thousand percent. I think he’s doing a great job. I think he’s got a great message.” He and Donald were photographed embracing on the night of the election.

The Post reported last year that a company in which Robert Trump has a financial stake received a $33 million federal contract.

Maryanne Trump Barry, the president’s sister, resigned last year as a federal appellate judge in the wake of an investigation into whether she violated judicial conduct rules related to her role in the family company’s tax practices.

The investigation, launched after the Times investigation into Trump family finances, became moot when she resigned. Trump once joked that he considered putting her on the Supreme Court. She has said little publicly about her brother during his presidency.

Trump’s other sibling, Elizabeth Grau, a former administrative assistant at Chase Manhattan Bank, has maintained a low profile throughout her brother’s presidency. She could not be reached for comment.