Caroline Kennedy wrote a scathing letter to key senators on Tuesday, calling her cousin, Robert F. Kennedy Jr., a “predator” addicted to attention from airing dangerous views on vaccinations and someone who is unfit to be the nation’s health secretary.

She urged lawmakers, who will be questioning Mr. Kennedy at his confirmation hearings Wednesday and Thursday, to reject his nomination. She cited his lack of experience, misinformed views on vaccines and personal attributes. In the letter, she described how he led other family members “down the path of drug addiction.”

“His basement, his garage, and his dorm room were the centers of the action where drugs were available, and he enjoyed showing off how he put baby chickens and mice in the blender to feed his hawks,” Ms. Kennedy wrote. “It was often a perverse scene of despair and violence.”

Her letter was first reported in The Washington Post.

Ms. Kennedy expressed particular outrage over the new disclosures in his ethics agreement filed with the Senate, which she described as outlining how his “crusade against vaccination has benefited him in other ways.”

She cited Mr. Kennedy’s decision to keep a financial stake in litigation against Merck, which makes a key vaccine against the human papillomavirus (HPV) that is administered to protect against cervical cancer.

“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,” Ms. Kennedy wrote.

As President Joseph R. Biden Jr.’s ambassador to Australia, Ms. Kennedy was actively involved in promoting the HPV vaccine, which has put Australia on a path to eliminate cervical cancer. She was instrumental in persuading Mr. Biden to expand his “cancer moonshot” initiative to the Indo-Pacific region.

In her role as ambassador, Ms. Kennedy said, she was reluctant to make public comments against Mr. Kennedy, who launched his presidential campaign in 2023 as a primary challenger to Mr. Biden before running as an independent candidate. When Mr. Kennedy dropped his presidential bid, he endorsed Mr. Trump, who, after winning the election, named Mr. Kennedy as his choice for health secretary.

After that, she broke with her cousin, saying his views about vaccination were dangerous.

Her letter painted Mr. Kennedy as a charismatic figure, “willing to take risks and break the rules,” and able to attract others through the strength of his magnetic personality. Then she traced a tragic history of Mr. Kennedy’s influence over other family members.

“But siblings and cousins who Bobby encouraged down the path of substance abuse suffered addiction, illness and death,” she wrote, “while Bobby has gone on to misrepresent, lie and cheat his way through life.”

Mr. Kennedy’s younger brother David died in Palm Beach County in May of 1984 of “multiple ingestion” of three drugs found in his body fluids, authorities said at the time.

Other relatives have also spoken out against Mr. Kennedy, including his brother Joseph Kennedy II and his sister Kerry Kennedy, who described his comments on race and vaccines as “deplorable and untruthful.”

On Tuesday, Jack Schlossberg, Ms. Kennedy’s son, who has also been critical of Mr. Kennedy, posted a video on social media of his mother reading the letter she had written.

“I’m so proud of my courageous mother, who’s lived a life of dignity, integrity and service,” Mr. Schlossberg wrote.

Ms. Kennedy, in the letter sent Tuesday, gave her cousin credit for overcoming his drug addiction, which Mr. Kennedy has discussed extensively. By his own account, Mr. Kennedy became addicted to heroin when he was 14, in 1968, as he struggled to cope with the assassination of his father. In 1984, he pleaded guilty to a felony charge of possessing heroin, and entered treatment.

But Ms. Kennedy was harsh in criticism of her cousin’s advocacy against vaccines, describing it as part of an addiction to attention and power.

“Bobby preys on the desperation of parents of sick children — vaccinating his own children while building a following by hypocritically discouraging other parents from vaccinating theirs,” she wrote.

Ms. Kennedy also highlighted “the conspiratorial half-truths he has told about vaccines,” in connection with the 2019 measles outbreak in Samoa, which she said “cost lives.”

The letter was addressed to senators who lead the committees that will be reviewing his nomination this week, including Mike Crapo, a Republican from Idaho; Ron Wyden, a Democrat from Oregon; Bill Cassidy, a Republican of Louisiana and Bernie Sanders, an Independent of Vermont.

She noted that the family is close and that speaking out was difficult. Still, she faulted her cousin for using the family’s legacy of tragedy for political gain. Mr. Kennedy’s father, Robert F. Kennedy, was assassinated during a campaign for president in 1968. Her father and his uncle, President John F. Kennedy, was fatally shot in Dallas in 1963.

Robert F. Kennedy Jr. “distorted President Kennedy’s legacy to advance his own failed presidential campaign — and then groveled to Donald Trump for a job,” the letter said. “Bobby continues to grandstand off my father’s assassination, and that of his own father.”

She suggested that her father John F. Kennedy, her uncle Robert F. Kennedy and another uncle, the long-serving lawmaker Ted Kennedy, “would be disgusted.”

She closed the letter with a plea for the senators to reject her cousin’s nomination on behalf of the doctors, nurses, scientists and caregivers who fuel the American health care system.

“They deserve a secretary committed to advancing cutting-edge medicine to save lives, not rejecting the advances we have already made,” Ms. Kennedy wrote. “They deserve a stable, moral and ethical person at the helm of this crucial agency. They deserve better than Bobby Kennedy — and so do the rest of us.”