Maryanne Trump Barry slammed her brother over his immigration policy that saw children separated from their parents at the border and sent to detention centers.
"All he wants to do is appeal to his base," she said in recordings obtained by the Washington Post. "He has no principles. None."
"His goddamned tweet and lying, oh my God."
The recordings were secretly made by the president's niece Mary Trump, who published last month a bombshell memoir of the "toxic family" that produced him.
The president's younger brother Robert, who died last week, went to court to try to block its publication, arguing that Mary was violating a non-disclosure agreement signed in 2001 after the settlement of her grandfather's estate, but to no avail.
Some 950,000 copies were sold the day of the memoir's release, with the White House branding the memoir as a "book of falsehoods".