Mary Trump is the daughter of Fred Trump Jr, brother of the current American president. Unlike other members of her family, Ms. Trump has sharply criticized the president on a number of issues.
Addressing the ITV information, Ms Trump insisted her presidential uncle was not a reliable ally of the UK.
She said: “For Britain to make certain decisions in the future, it must know that it can count on the United States as an ally.
“I don’t think you can do it right now, in a deep way, like you have in the past. “
The Trump family tried to block publication of the book, but it was rejected by a judge on July 14.
Mary Trump has lost her father, President Trump’s older brother, just 16, to an alcohol-related illness.
Fred Trump Jr has struggled to drink much of his ilk and President Trump has already linked this to his campaign against opioid addiction since taking office.
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She added: “He has grown into an adult unable to look outside his own interests.
“He’s basically stuck telling himself a story every minute, he’s the best, he’s the greatest.
“He always wins because if he loses the thread of this story, he will have to face things about himself that would be unbearable for him.
“He must therefore engage in numerous projection and defense mechanisms to protect himself from it, the reality that he is nothing of what he claims to be.
“I believe that the story I am telling has a lot of explanatory power and that people will benefit from reading it.”
President Trump has furiously denied that the book provides an accurate account of his youth.
The book, published by New York-based Simon & Schuster, currently tops Amazon’s bestseller list.
In her book, Ms. Trump, who has a doctorate in clinical psychology, accused her uncle of being narcissistic and of believing that “nothing is ever enough”.
She wrote, “It’s way beyond the narcissism of garden varieties.
“Donald is not just weak, his ego is a fragile thing that must be strengthened every moment because he knows deep down that he is nothing of what he claims to be. “