NEW YORK — Mary L. Trump, a niece of the president who wrote a scathing bestseller documenting deep family dysfunction, has sued President Trump and two of his siblings, alleging they defrauded her out of tens of millions decades ago by manipulating the value of properties and lying to her about the worth of her inheritance.

The lawsuit, filed Thursday in New York Supreme Court, accuses Mary Trump’s uncles, Donald and Robert, and aunt Maryanne Trump Barry, of pretending they would guard her assets — valuable pieces of the Trump family real estate empire — that she inherited after her father died, only to bully and take advantage of her during estate settlement negotiations. It alleges fraud, conspiracy and violations of fiduciary duties.

The White House did not respond to a request for comment. At a briefing Thursday, press secretary Kayleigh McEnany deflected a reporter’s question about the allegations, commenting instead on a secret recording the president’s estranged niece made of Maryanne Trump Barry in which the retired federal judge was heard insulting the president.

“The only fraud committed there,” McEnany said, “was Mary Trump recording one of her relatives, and she’s really discredited herself.”

A person who answered the phone at Maryanne Trump Barry’s home said she was unavailable.

Robert Trump died last month. The lawsuit names his estate, which was valued at $50 million, according to court records. An attorney handling Robert Trump’s estate and the estate’s proposed executor did not respond to requests for comment.

Mary Trump’s father, Fred Trump Jr., was the oldest of President Trump’s siblings. He died in his 40s when she was a teenager. Her three older relatives “committed to watch over [Mary Trump’s] interests as fiduciaries,” the lawsuit says. “They lied.”

A trained psychologist, Mary Trump alleged in her book, “Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man,” that the president and his siblings swindled her and her brother when the estate of her grandfather Fred Trump Sr. was litigated and settled after his death in 1999.

According to the lawsuit, the Trump siblings took control of their father’s massive real estate portfolio in his declining years, as he was suffering from dementia. It alleges they siphoned money from him before his death, and then filtered funds through a sham company and other means to reduce their inheritance tax liability.

The lawsuit, based primarily on information detailed in a 2018 New York Times report, accuses the defendants also of wrestling away power of attorney from Fred Trump Sr., enabling them to direct their father’s business interests as they wished.

The defendants “devalued” properties that included Mary Trump’s financial interests, which minimized the worth of her assets and ultimately affected what she was paid, the lawsuit alleges. It claims she was then misled by her family with the assistance of her trustee, Irwin Durben, a longtime family attorney whose true loyalty was to the Trumps who were running the business. Durben died in 2016, the suit says.

“Rather than protect Mary’s interests, [her uncles and aunt] designed and carried out a complex scheme to siphon funds away from her interests, conceal their grift, and deceive her about the true value of what she had inherited,” according to the civil complaint.

It is unclear why the lawsuit was filed now, but Mary Trump is probably approaching a statute-of-limitations deadline. The New York Times investigation published two years ago next month, and fraud claims typically need to be filed within two years.

The complaint does not say how much money Mary Trump ultimately inherited under the terms of a sealed settlement agreement, or how much she was allegedly shorted. Her lawyer, Roberta Kaplan, said her client lost “tens of millions” but declined to specify further.

Mary Trump has said previously that her uncle is unfit for office and that she would do “everything in my power” to see that his Democratic opponent, former vice president Joe Biden, is elected in November.

Michael Kranish in Washington contributed to this report.