Maryanne Trump Barry, the eldest sister of ex-President Donald Trump, has died at the age of 86, US media reports.
She was found dead at about 04:00 EST (09:00 GMT) on Monday morning in her New York City apartment, according to ABC News. Her cause of death has not yet been revealed.
Mrs Barry served as a federal judge in New Jersey from 1983 until her retirement in 2019.
She is the third of Mr Trump's four siblings to have passed away.
A spokesman for the former president did not immediately respond to the BBC's request for comment.
Sources told ABC that emergency crews had responded to a call of a person in cardiac arrest. There were no signs of trauma or foul play, the outlet added.
A medical examiner will later determine cause of death.
Mrs Barry, who eschewed the family's real estate business, served as one of only two female prosecutors in the US Attorney's Office in New Jersey from 1974 to 1983.
With the help of her brother's infamous lawyer and fixer Roy Cohn, Mrs Barry was nominated by President Ronald Reagan as a federal district judge in 1983.
In "Too Much and Never Enough: How My Family Created The World's Most Dangerous Man", a 2020 memoir written by her niece Mary, Mrs Barry said it was "the only favour I ever asked for in my whole life".
It was a favour, though, that she claimed Mr Trump never let her forget.
"Where would you be without me?" she quoted him as saying.
In 1999, Mrs Barry was appointed by President Bill Clinton to the US Court of Appeals for the Third Circuit, which encompasses portions of New Jersey, Delaware and Pennsylvania.
By 2019, a New York Times investigation of the Trump family's tax affairs had led to a judicial misconduct inquiry into Mrs Barry. She retired that February and the inquiry was closed without conclusion.
Mrs Barry married twice - first to David Desmond, with whom she had one son, also named David; and then to John Joseph Barry, who died in 2000.
She was a close confidante of her younger brother, who - while first running for president in 2015 - said she would be a "phenomenal" choice if ever considered for the US Supreme Court.
But in 2016, he acknowledged in a radio interview that the pair "have different views a little bit".
A Republican like Mr Trump, Mrs Barry shunned the limelight and never spoke publicly about him.
But their relationship is said to have taken a turn following the release of Mary Trump's book, which drew in part from conversations between Mrs Barry and her niece.
As she promoted the book, Ms Trump, an outspoken critic of her uncle, released audio excerpts from these exchanges that she had secretly taped.
"All he wants to do is appeal to his base," Mrs Barry is heard saying in one recording shared by the Washington Post. "He has no principles. None."
Mrs Barry is also heard telling her niece that she "did [Mr Trump's homework for him" and that when "he got into University of Pennsylvania he had somebody take the exams".
But the siblings were beginning to mend fences together and had been seen together this summer at the former president's club in Bedminster, New Jersey, according to ABC.
Mr Trump, who is the frontrunner for the Republican presidential nomination, lost his brothers Fred and Robert in 1981 and 2020 respectively. He also lost his first wife, Ivana Trump, last year.
His only surviving sister, Elizabeth Trump Grau, 81, is a former banker.