Mary Trump, the niece of the president, dissects what she says was Donald Trump’s emotionally abusive childhood, in her new book out next week. She writes that neglect from his parents - his mother because of health problems and his father because of being a “high functioning sociopath” fuels Donald Trump’s outrageous behavior now, and threatens “the world’s health, economic security, and social fabric
Mary Trump's book, Too Much and Never Enough: How My Family Created the World's Most Dangerous Man, describes her uncle as a fraud and a bully.
The White House rejects claims made in the book, excerpts of which have been leaked to US media.
The Trump family has sued to block its 14 July publication.
'More than narcissism'
Ms Trump, 55, writes that for her uncle, "nothing is ever enough" and that the US president exhibits all the characteristics of a narcissist.
"This is far beyond garden-variety narcissism," his niece, who has a doctoral degree in clinical psychology, writes of Mr Trump. "Donald is not simply weak, his ego is a fragile thing that must be bolstered every moment because he knows deep down that he is nothing of what he claims to be."
She says the president was influenced by watching his father, Fred Trump Sr, bully her father Fred Trump Jr - who died from an alcohol related illness when she was 16 years old.
Ms Trump writes that Trump Sr was extremely harsh to his oldest son, whom he wanted to take over the family real estate business. But as Ms Trump's father drifted away from the business, Trump Sr had no choice but to turn to his second son, Donald.
It was not a happy choice, Ms Trump appears to claim. "When things turned south in the late 1980s, Fred could no longer separate himself from his son's brutal ineptitude; the father had no choice but to stay invested," she writes of the senior Trump's attitude towards the future 45th US President.
The White House rejected the claim that Mr Trump's father had been abrasive and harsh, saying that the president "describes the relationship he had with his father as warm and said his father was very good to him".
'I had to take Donald down'
In the book, Ms Trump describes how she supplied tax documents to the New York Times, which used them to publish a 14,000 word investigative article into Mr Trump's "dubious tax schemes during the 1990s, including instances of outright fraud, that greatly increased the fortune he received from his parents".
Ms Trump said she was approached by journalists at her home in 2017 and was initially reluctant to help.
She waited for a month, watching as "Donald shredded norms, endangered alliances, and trod upon the vulnerable," before deciding to contact the Times reporter.