Donald Trump Jr.'s niece called Donald Trump Jr. a "suck-up" and her "stupidest".
Mary Trump stated that her cousin would "outracism anyone" to gain approval from the Trump family.
Former President Trump is suing her over a book she wrote about him.
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In a podcast interview, Mary Trump called Donald Trump Jr. "deeply inintelligent" as well as "weak."
"I've been asked the following: Who's the dumbest one? And it's him," replied the psychologist, who is being sued her uncle, ex-President Donald Trump, for publishing a book that criticizes him on a Wednesday episode "UnPresidented."
Mary Trump said that her cousin was "left behind" by her father, who prefers Ivanka.
"And yet, he still attempts, doesn't? Mary Trump. He's just like other members of my family, I think. He doesn't have a core, ideology, or beliefs. He'll out-racise anybody, he will out-misogyny anyone, and he'll kill as many innocent animals to get affection in my family.
She said that Donald Trump Jr. was the oldest of his siblings and therefore he was expected to follow his father's lead and surpass him.
She said, "He didn't intend to strike out on his own. He had no intention other than sucking it up. Toeing the line."
She also took aim at Donald Trump, calling him "the weakest person in the world."
Mary Trump is a fierce critic of former President Donald Trump and his children. She called her uncle a fascist and demanded that he be sent to prison.