Trump was twisted by his 'sociopath' father and his own sister thinks he's a 'clown' -- according to his niece

Donald Trump's niece describes the US president as a lying narcissist who was shaped by his domineering father, according to excerpts of her eagerly anticipated memoir carried in US media Tuesday.

Mary Trump's "Too Much and Never Enough: How My Family Created the World's Most Dangerous Man" is due out next week amid a legal battle to stop its publication, and is already a best-seller on Amazon.

In it, she accuses Trump of "hubris and willful ignorance" stretching back to his younger days, according to CNN, which has seen a copy.

She writes that Trump developed "twisted behaviors" and saw "cheating as a way of life," according to The New York Times.

Trump alleges that the future US leader cheated on the SAT pre-collegiate exam, helping him get into the University of Pennsylvania's prestigious Wharton business school, the newspaper reported.

She says her uncle paid someone else to take the test when Trump was a high school student in New York. The Times doesn't explain how she knew.

The 240-page book, out July 14, says Trump is a product of his "sociopath" father Fred Trump, The Washington Post reported.

Mary, a clinical psychologist, says her uncle failed to develop human emotion because his father created an abusive and traumatic home life.

She says that for the future US leader, "lying was primarily a mode of self-aggrandizement meant to convince other people he was better than he actually was," the Post said.

The memoir is billed as the first unflattering portrayal of Trump by a family insider.

The president's younger brother Robert Trump went to court to try to block publication, arguing that Mary was violating a non-disclosure agreement signed in 2001 after the settlement of her grandfather's estate.

Last week, a New York appeals judge ruled that Simon & Schuster is allowed to release the memoir, saying it was "not a party to the agreement."

On Tuesday, White House spokeswoman Kayleigh McEnany described it as "a book of falsehoods," even though she admitted she hadn't seen it.

"It's ridiculous, absurd allegations that have absolute no bearing in truth," McEnany told reporters.

- 'Clown' -

Mary is the daughter of Fred Trump Jr, Trump's older brother, who died in 1981 from complications related to alcoholism. Fred Trump Sr died in 1999.

She writes that her uncle meets all the clinical criteria for being a narcissist, according to The New York Times.

"Donald's pathologies are so complex and his behaviors so often inexplicable that coming up with an accurate and comprehensive diagnosis would require a full battery of psychological and neurophysical tests that he'll never sit for," she writes.

The memoir is also due to reveal that Mary was the crucial source for explosive New York Times reporting on Trump's finances, which suggested the billionaire paid little in tax for decades, according to The Daily Beast.

In the book, she quotes the president's sister Maryanne Trump Barry as "saying he's a clown -- this will never happen," after Trump announced he was running for president.

It is set to be the latest bombshell book to dish dirt on Trump after former aide John Bolton's tome, which describes the Republican leader as corrupt and incompetent, hit shelves last month.