In a letter on Tuesday to Senators, Ms. Kennedy - who previously served as a US ambassador to Australia and Japan and is the daughter of former President John F Kennedy - etched a damning sketch of the her cousin, the latest condemnation Kennedy Jr has received from members of his own, prominent Democratic family.
Several denounced her cousin's bid for the presidency in 2024. The letter, obtained by The Associated Press, was first reported by The Washington Post.
Caroline Kennedy believes her late father and uncle would be disgusted by Robert F Kennedy Jr. (Lukas Coch/AAP PHOTOS)
Ms Kennedy said in the letter that her cousin's views on vaccines are disqualifying. She offered senators personal details from their lives growing up together that she said pose an even greater concern.
She described her cousin's basement, garage and dorm as being an epicentre for drug use, where he would also put baby chickens and mice in blenders to feed to his hawks.
"It was often a perverse scene of despair and violence," she wrote. She also read the letter in a video recording, sharing it on social media. Attempts to reach Kennedy Jr for comment were not immediately successful.
He has openly talked about his heroin use and he pleaded guilty for bringing it on a plane in 1984.
Kennedy Jr now "preys on the desperation of parents of sick children," she told senators, noting that he has vaccinated his own children while discouraging others from vaccinating theirs.
She also pointed out that Kennedy Jr plans to still profit off a lawsuit against pharmaceutical company Merck over Gardasil, its human papillomavirus vaccine that prevents cervical cancer. In 2024, he made over $US850,000 ($A1.4 million) from the arrangement.
"In other words, he is willing to enrich himself by denying access to a vaccine that can prevent almost all forms of cervical cancer and which has been safely administered to millions of boys and girls," she wrote. She had previously not spoken about his candidacy or nomination for health secretary.
Her father was assassinated in 1963 when she was a young girl. Kennedy Jr's father was assassinated in 1968 during his presidential campaign event.
"Unlike Bobby, I try not to speak for my father – but I am certain that he and my uncle Bobby, who gave their lives in public services, and my uncle Teddy, who devoted his Senate career to improving health care, would be disgusted," Ms. Kennedy wrote.
Kennedy Jr will appear before the Senate Finance Committee on Wednesday in his bid to become Trump's health secretary. Another hearing for his nomination will be held on Wednesday in front of the Health, Education, Labor and Pension, the committee his uncle, Ted Kennedy, once chaired.
