Fred Trump III kept silent for years about his uncle, former president Donald Trump. And when his sister, Mary L. Trump, wrote a 2020 bestseller calling their uncle “the world’s most dangerous man,” he called the book “a breach of trust and a violation of our privacy,” adding that “We consider our family matters to be private.”

Yet Tuesday marked the release of Fred Trump III’s own book, in which he does an about-face to take on his uncle and recount a deeply dysfunctional extended family.

The turning point, Fred Trump III said, was an Oval Office conversation about his son William, who has physical and mental disabilities, in which the president said about people with disabilities, “The shape they’re in, all the expenses, maybe those kinds of people should just die.”

That was the last time he talked with his uncle, Fred Trump III said in an interview Tuesday with The Washington Post, setting him on the path to write “All in the Family: The Trumps and How We Got This Way.” He said he planned to vote for the likely Democratic nominee, Vice President Harris, and that, if asked, he would campaign against his uncle and speak at the Democratic National Convention.

“I have stayed quiet for a very long time,” said the 61-year-old Fred Trump III. “I’m one of the only people that knows Donald from his formative years to his business career to the White House.”

With that background, he said, he set out to write “a three-dimensional book. It’s the good, the bad and the jaw-dropping.”

Former president Trump, in a statement to The Post, said he had helped Fred Trump III, contributed to a fund to help William Trump, and also introduced Fred Trump III to government officials from the National Institutes of Health and elsewhere who might be helpful.

“I helped him so much, more than anyone else in his life, and this is the thanks I get. I even set up [a] meeting with NIH and top doctors in the country talking about his son, who is disabled,” Donald Trump said in the statement. He said that Mary Trump “convinced him to do this.”

But Fred Trump III said that he has not talked to Mary since her book published. She could not be reached for comment.

Donald Trump and his spokesman did not respond directly to a question about Fred Trump III’s assertion about his comments regarding disabled people.

Fred Trump III said the book is, by design, a nuanced and somewhat gentler view than his sister’s volume. He recounts the many disputes within the family but saved the toughest comments about Donald Trump and his views on people with disabilities until the final pages. That episode fits his conclusion that Donald Trump has an innate lack of empathy.

The backstory to both Mary and Fred Trump III’s books is Donald Trump’s relationship with their father — his older brother — Fred “Freddie” Trump Jr., who died in 1981 of an alcoholism-related illness at age 42.

Donald Trump, who rarely admits mistakes, told The Post in 2019 that he regrets having tried to press his brother to abandon his work as a TWA pilot and join the family company, telling Fred Jr. “you’re wasting your time.” Donald Trump said in The Post interview that “I do regret having put pressure on him.”

Fred Trump III, asked in the interview whether he blames Donald Trump’s pressure for contributing to his father’s struggles, responded, “Yes, I do,” saying that “my father became an airline pilot flying a 707. You know, in those days, pilots were sort of heroes along with the astronauts.” Instead of accepting his brother’s decision, “Donald continued to beat down my father. And that’s unfortunate,” Fred Trump III said.

When the family patriarch, Fred Trump Sr., died in 1999, years after being diagnosed with dementia, Fred Trump III and his sister, Mary Trump, expected they would receive a portion of the estate that would have gone to their father.

Instead, after Fred Trump III and Mary Trump were told that they would be getting far less than they expected if there had been an equal division of the estate, they sued Donald Trump and two of his siblings, alleging that they had taken advantage of Fred Trump Sr.’s dementia and gotten him to change the will.

Donald Trump and his siblings denied the allegation and responded by cutting off family medical insurance that was crucial for the care of Fred Trump III’s son. Donald Trump said at the time to the New York Daily News, “When [Fred Trump III] sued us, we said, ‘Why should give him medical care?’”

The feud escalated as Fred Trump III said in a court case that Donald Trump and his siblings “thought nothing about taking away my critically ill son’s coverage in an attempt to browbeat me in to abandoning my claim in the probate contest.”

Fred Trump III and his sister, Mary, eventually settled their suit under a confidentiality agreement.

With the publication of Mary’s book, “Too Much and Never Enough: How My Family Created The World’s Most Dangerous Man,” the first public crack appeared among the Trumps, but no one else in the family joined her criticism. She based her book in part on secret tape recordings of her aunt Maryanne Trump Barry, first reported in 2020 by The Post, in which Donald Trump’s sister said her brother has “no principles” and that “you can’t trust him.”

Fred Trump III, who often visited Barry, wrote in his book that she told him in 2016 that she was writing an op-ed urging people to vote against Donald Trump, but the opinion piece was never published for reasons that are not known. Fred Trump III recalled seeing Barry writing the op-ed on a yellow legal pad and that it said “he’s just not fit to be president.” Barry died in 2023.

Fred Trump III said in the interview that, to this day, he has not read his sister’s book.

While the relationship between Mary and Fred Trump III frayed, he stayed in regular touch with Donald Trump after his 2016 election, visiting him about a dozen times at the Oval Office, he said in the interview, despite differing politics. Fred Trump III said he voted for Democrats Hillary Clinton in 2016 and Joe Biden in 2020 — but refrained from publicly speaking out against his uncle.

He said that Donald Trump voluntarily and regularly contributed an undisclosed amount to a fund to help pay for his son’s medical expenses.

Donald Trump, meanwhile, said in his 2019 interview with The Post that his dispute with Fred Trump III was in the past. “One child was having a difficult time,” Donald Trump said of William. “It was an unfortunate thing. It worked out well, and we all get along.”

But the rapprochement between the former president and his nephew collapsed with Tuesday’s publication of Fred Trump III’s book.

Fred Trump III said that he hopes the book will spark a national conversation about helping families of those with disabilities, including the difficulty of caring for adult children. William is now 25 years old, lives in a group home and is doing relatively well medically, Fred Trump III said, but “we’re still struggle every day” to care for him.

To this day, Donald Trump has never met William, according to Fred Trump III. He believes that is due to a view that anyone “below him” is not a worthy person “unless he needs their vote.”