Mary Trump will say she supplied The Times with confidential family tax documents, The Daily Beast said.
President Donald Trump's niece is set to publish a tell-all book on their family drama in August,
in which she will say she leaked tax documents to help a New York Times investigation into his finances, according to The Daily Beast.
Mary Trump, 55, is the daughter of Fred Trump Jr., the president's older brother who died in 1981 at the age of 42 from a heart attack triggered by alcoholism.
Her book, "Too Much and Never Enough," is scheduled to arrive August 11 from Simon & Schuster,
three months ahead of the presidential election and days before the Republican National Convention.
People with knowledge of her book told The Daily Beast that it would be filled with "harrowing and salacious" stories about the president.
According to The Daily Beast, she will identify herself as the primary source for The New York Times' 2018 Pulitzer Prize-winning exposé on the president's finances by supplying her grandfather's tax returns and other confidential financial papers.
The Times investigation had revealed that the president wasn't a self-made man but had his father bolster his failing businesses in the 1990s with injections of more than $400 million in today's dollars.
Mary Trump's book will also allege that Donald Trump and his father played a role in his brother's death by neglecting him when he needed their help, The Daily Beast reported.
The sources said the book would also include conversations with the president's sister, Maryanne Trump Barry, a retired federal judge who is said to express "damning thoughts about her brother," according to The Daily Beast.
There has been bad blood between Mary Trump and her uncle Donald since the death of Fred Trump Sr. in 1999.
Mary Trump and her brother, Fred Trump III, contested Fred Trump Sr.'s will because he split most of his wealth between his four surviving grandchildren and failed to give Mary and Fred what would have been their father's share of the inheritance.
They did, however, receive the same $200,000 given to each surviving grandchild.
In response, Trump and two of his siblings tried to cut the two out of the family's medical-insurance plan — which was a huge blow for Fred Trump III,
who has a son with cerebral palsy who requires extensive care.
Mary Trump gave a rare interview about the case to the New York Daily News in 2000, saying her "aunt and uncles should be ashamed of themselves,
" referring to Donald and his brothers, and Maryanne Trump Barry.
Donald Trump told the outlet at the time that his father discounted Mary Trump and Fred Trump III's inheritance because he had a "tremendous dislike" for their mother,
Linda Clapp. The couple's marriage was short-lived.
"I think he felt if it goes to the two children, it also maybe can go to the mother indirectly," he said. "He felt the mother was the cause of some of Fred's difficulty, and Fred had a difficult life."
The president has spoken frequently about his older brother's struggle with alcoholism on several occasions, saying it's the reason he doesn't drink.
Fred Trump Jr. was once his father's heir apparent but eventually quit the family business to become a pilot, resulting in his younger brother rising through the ranks.
Last year, Trump told The Washington Post that he regretted pressuring his brother to take more of an interest in the family business.
He stopped short of accepting responsibility for his brother's death, however, saying: "I don't think there was much we could do at the time."
