Last week, Hines was indeed rattled by Kennedy and added her voice to an avalanche of others, including many in his immediate family and among Jewish advocacy groups, condemning Kennedy’s comments implying that people who oppose the COVID-19 vaccine are being persecuted more severely than Anne Frank, who died in a Nazi concentration camp.

In a tweet, Hines called her husband’s remark at an antivaccine rally last Sunday in Washington “reprehensible.” Kennedy’s sister Kerry Kennedy likewise denounced his “hateful rhetoric,” describing it as “sickening and destructive.” And his niece Kerry Meltzer, a physician in New York, called his statements “abhorrent.”

Kennedy has since offered a qualified apology — “To the extent my remarks caused hurt, I am truly and deeply sorry,” he tweeted. But the controversy is merely the latest to embroil the 68-year-old son of Senator Robert F. Kennedy and nephew of President John F. Kennedy. His increasingly extreme views on vaccinations and embrace of conspiracy theories have caused a deepening rift in one of America’s most prominent political families.

In addition to Kennedy’s incendiary rhetoric about vaccines, many were stunned last month when, over the objections of his 93-year-old mother, Ethel Kennedy, he argued that Sirhan Sirhan, the man convicted of assassinating his father in 1968, should be released from prison because, Kennedy claims, he didn’t do it. And Kennedy previously raised eyebrows with his zealous defense of Michael Skakel, a cousin who was convicted in the 1975 death of teenager Martha Moxley. (Kennedy wrote a best-selling book about the case claiming Skakel was framed.) But in the midst of a pandemic that has killed more than 875,000 people in the United States, it’s Kennedy’s strident antivaccination crusade that his family and others find most alarming.

“To make a comparison with the Nazi era is not something his father would have ever done,” said Greg Payne, chair of the department of communication studies at Emerson College and a volunteer on Robert F. Kennedy’s 1968 presidential campaign. “I think Robert F. Kennedy would say [to his family] we need to be the voice of reason. Unfortunately, Robert F. Kennedy Jr. is not that voice.”

Kennedy, who was 14 when his father was murdered in 1968, has spent much of his professional life as a lawyer and environmental activist. The scion of the dynastic clan has had an occasionally tumultuous private life. He graduated from Harvard in 1976 and earned a law degree at the University of Virginia. In 1983, while working in the Manhattan district attorney’s office, he was arrested for possession of heroin and sentenced to community service. Hines is Kennedy’s third wife. He and his first wife, Emily Black, were together for 12 years and had two children before divorcing in 1994. That same year, Kennedy married Mary Richardson, with whom he had four children. Richardson died by suicide in 2012, two years after Kennedy filed for divorce.

Kennedy has never held elected office, but did briefly consider running for the New York Senate seat once held by his father. In 1999, he founded the nonprofit Waterkeeper Alliance to protect the country’s waterways from pollutants, and for several years he also taught environmental law at Pace University. That partnership, however, has ended.

“Pace University is not affiliated with Mr. Kennedy, nor do we endorse his public statements,” a university spokesperson wrote in an e-mail last week.

Long before his wildly overwrought rhetoric about the dangers of the COVID jab, Kennedy was asserting, contrary to all credible scientific evidence, that the vaccine for measles, mumps and rubella (MMR) causes autism and other neurological disorders, and claiming that public health officials “knowingly allowed the pharmaceutical industry to poison an entire generation of American children.”

It was an outrageous assertion debunked by the Centers for Disease Control and Prevention, the World Health Organization, and the American Academy of Pediatrics, among others, but it nonetheless found a large audience. One person paying attention was former president Donald Trump, who, soon after taking office in 2017, appointed Kennedy to chair a “vaccine safety commission,” a move that disturbed public health officials. It’s not clear if the commission ever met.

As Kennedy’s antivaccine foundation, Children’s Health Defense, gained followers and influence, his family grew more and more uncomfortable. It was, after all, his uncle, President Kennedy, who signed the 1962 Vaccination Assistance Act, which sought to protect preschool children through mass immunizations.

“We love Bobby,” Kathleen Kennedy Townsend and Joseph P. Kennedy II, two of Kennedy’s 10 siblings, wrote in Politico in 2019 amid a resurgence of measles. “However, on vaccines he is wrong. He has helped to spread dangerous misinformation over social media and is complicit in sowing distrust of the science behind vaccines.”

No member of the Kennedy family responded to an interview request for this story.

While Kennedy has enormous followings on Facebook and Twitter, his Instagram account was removed last year for “repeatedly sharing debunked claims” about COVID vaccines, and researchers at the Center for Countering Digital Hate have identified Kennedy as one of its “Disinformation Dozen” — the 12 people responsible for the bulk of the false or misleading claims about COVID vaccines on social media.

Still, Kennedy isn’t backing down. Indeed, despite CDC findings that unvaccinated people are 14 times more likely to die of COVID than those who get the shot, Kennedy continues to warn against the jab, calling it, wrongly, the “deadliest vaccine ever made.”

“He’s gone off the rails and no one can seem to control him,” said a longtime Kennedy friend who asked not to be identified. “It’s nonsense and it’s frightening.”

His family’s disapproval notwithstanding, Kennedy has used his famous name and the Kennedy mystique to solicit donations to Children’s Health Defense, even offering supporters a chance to visit the family’s compound on Cape Cod.

“You and your guest will join me for a day of sailing and private tour of the legendary Kennedy Compound in Hyannis Port,” he wrote in a promotional post in 2019 that featured a photo of Kennedy family members on a wooden sailboat. “The more you contribute, the greater your chances of winning!”

In fact, the pandemic, and Kennedy’s response to it, has been a boon to Children’s Health Defense, which, according to an investigation by the Associated Press, has seen its revenues grow from $1.1 million in 2019 to $6.8 million in 2020. Its misinformation is also being widely consumed, with 4.7 million visits to its site last August, according to the AP, up from just 119,000 monthly visits before the pandemic.

Kennedy’s inflammatory comment about Anne Frank was not the first time he’s likened the antivaccination movement to victims of the Holocaust. In one of his many online posts complaining about being censored, Kennedy included a photo of a group of men making the Nazi salute while standing around a pile of burned books.

“If you say anything on social media that questions the government, you will be expelled,” Kennedy said last week in an interview with Spin magazine about his new book, “The Real Anthony Fauci,” a conspiratorial critique of the government’s top infectious disease expert. “It has nothing to do with factual accuracy. It’s totalitarianism.”

But some public health experts believe Kennedy’s antivaccination views have been amplified because of his influential family. Who, after all, would heed him but for his name?

“He calls people in the government or at the [National Institutes of Health] and he gets those calls answered,” said David Gorski, a cancer surgeon at Wayne State University School of Medicine and critic of the antivaccination movement. “He uses his family’s position as a liberal icon as a very effective wedge to further what is now pretty much a right-wing movement.”

Kennedy is a frequent guest on Fox News shows and has appeared at events alongside Trump allies Roger Stone, former national security adviser Michael Flynn, and Charlene Bollinger, who, with her husband, Ty, has peddled millions of dollars’ worth of antivaccine products.

David Nasaw, a retired City University of New York history professor who has studied the family extensively, traces Kennedy’s rampant skepticism of government and science to the assassinations of his father and uncle — events that have inspired many tomes based on fanciful or baldly false claims.

Nasaw said he interviewed Kennedy in 2011 for a book about his grandfather, Joseph P. Kennedy Sr. At the end of their meeting, Kennedy handed him a book titled “JFK and the Unspeakable,” which concludes that the president was murdered by his own security apparatus. The assassination has been investigated time and again, and no killer other than Lee Harvey Oswald has ever been credibly identified.

“This type of thinking penetrated Kennedy early on and just built and built and built over the years into an almost uncontrolled paranoia that all authorities — the government, the scientists — are lying, always,” said Nasaw.

But it’s not only his antivaccination bombast that bothers those closest to him. Friends say Kennedy’s elderly mother was stricken by her son’s recent appeal for the release of Sirhan, who has served 53 years in prison for shooting Robert F. Kennedy.

In an op-ed published last month by the San Francisco Chronicle, Kennedy described Sirhan as “gentle, humble, kindhearted, frail, and harmless,” and said he was “bullied into conceding guilt.”

Kennedy’s mother responded with a statement of her own about Sirhan, saying “our family and our country suffered an unspeakable loss due to the inhumanity of one man. . . . We believe in the gentleness that spared his life, but in taming his act of violence, he should not have the opportunity to terrorize again.” In the end, Sirhan was not paroled.

It was a victory for the family, but they seem powerless now to prevent Robert F. Kennedy’s namesake from continuing to foment antivaccine fervor.

“It’s very painful for Ethel,” said a Kennedy confidante, “the whole thing.”

Mark Shanahan can be reached at mark.shanahan@globe.com. Follow him on Twitter @MarkAShanahan. Hanna Krueger can be reached at hanna.krueger@globe.com. Follow her on Twitter @hannaskrueger.