Robert F. Kennedy Jr’s sister has become the face of the Kennedy family effort to block his candidacy and re-elect President Biden. She has more at stake besides her relationship with her brother.

When members of the Kennedy family joined President Biden in Philadelphia to endorse his re-election — and denounce the presidential candidacy of the best-known Kennedy of this generation, Robert F. Kennedy Jr. — one person stepped forward to make the family’s case: his younger sister Kerry.

“Nearly every single grandchild of Joe and Rose Kennedy supports Joe Biden,” Ms. Kennedy said as her siblings, and Mr. Biden, flanked her onstage. “That’s right: The Kennedy family endorses Joe Biden for president.”

That was not the first time that Ms. Kennedy, the seventh child of Robert and Ethel Kennedy, had been the face of the family’s pushback against her brother. As Mr. Kennedy has emerged as a skeptic of Covid-19 vaccines and a purveyor of conspiratorial theories on the assassination of his father, it has fallen to Ms. Kennedy to distance her family from the brother she has long held close and to guard the legacy of a proud and private family as it fades from the political stage.

To a large extent, Ms. Kennedy’s siblings say, her outsize role is an outgrowth of the affection she has displayed toward her brother since they were children playing on the grounds of the family estate in Hickory Hill, Va. — and the disappointment she feels now. It is political as well: She argues that her brother’s insurgent campaign threatens the re-election of Mr. Biden and is aware that her family could shoulder some of the blame should Donald J. Trump return to the White House next year.

“I love Bobby,” she said in an interview. “It’s heart-wrenching to be in this position.”

But Kerry Kennedy’s decision to help lead an inevitably awkward effort against a family member also reflects her concern about the legacy of Robert F. Kennedy Human Rights, the organization founded upon her father’s death in 1968 that she has led for 15 years.

As its president, she has championed causes that reflected her father’s work: international human rights, fighting against disparities in health care and advocating for immigrants. In some cases, the positions that have been embraced by her brother, particularly his opposition to Covid vaccination, have run directly counter to the mission of the organization.

“Kerry feels a special burden with his candidacy — a burden that impacts her work in a negative way,” said Christopher G. Kennedy, one of their brothers. “She has supported hundreds of human rights activists around the world. Her abilities would be diminished if the Kennedy name is associated with fringe thinking, crackpot ideas and unsound judgment.”

Robert F. Kennedy Jr. declined to comment on his sister’s role in the Biden campaign. “I think I’ll stay out of this controversy,” he said in a text message.

It has been 56 years since Robert F. Kennedy was assassinated in Los Angeles. But Ms. Kennedy, 64, said that many people, particularly younger generations, no longer distinguished between a father and a son who share the same name.

“Anybody can go look at the responses on my X feed or Instagram and see there’s a lot of confusion out there,” she said in the interview. “I have been compelled to clarify when the organization differs on issues such as the safety of vaccines, the role of H.I.V. in causing AIDS, the reason people are transgender and the role of platforms curbing disinformation, to name a few.”

Ms. Kennedy’s view on the potential impact of her brother’s candidacy is not shared across the family. “I think the mistake the media makes is to think that his appeal to people is solely based on his relationship to our family,” said Douglas H. Kennedy, another of their brothers, who has not joined his siblings in the pushback because, he said, of his work as a correspondent on Fox News. “I believe his followers are unaffected by his siblings’ or cousins’ concerns. His supporters believe in his message. I don’t think people coming out against him will have a great effect on that support.”

The Biden campaign disagrees. It is moving to deploy Ms. Kennedy, as well as other family members, in swing states in the coming months if her brother continues to make it onto ballots. “I’ve told the Biden campaign that I’ll campaign wherever they want me to go,” she said.

Biden campaign officials said their polling showed that many people do not know much about Mr. Kennedy or what he stands for, and that Kennedy family members are particularly effective at filling in the gaps.

“We’re honored to have the Kennedys’ support, and we look forward to working with them to spread the message on the campaign trail about how the president is carrying on the Kennedy legacy,” Lauren Hitt, a campaign spokeswoman, said.

Ms. Kennedy is no stranger to being in the spotlight. Her marriage to Andrew M. Cuomo in 1990, before he was elected governor of New York, was a merger of two of the nation’s most famous political families. It took place in the church in Washington, D.C., where John F. Kennedy’s funeral was held in 1963 as camera crews and photographers waited outside. That union ended in divorce 15 years later, the details of the breakup strewn across the tabloids. She never remarried.

Seven years after the end of the marriage, Ms. Kennedy was arrested and charged with driving while under the influence in northern Westchester County, N.Y. She said she had mistakenly taken a single sleeping pill before entering her car. A jury took less than an hour to find her not guilty, but the case made her life tabloid fodder once again.

Ms. Kennedy endured but largely stayed out of the public spotlight — unlike recently.

Her earnest appeal for Mr. Biden — the timbre of her voice familiar to anyone who watched her father or uncle — has offered a contrast to a series of mocking Instagram posts by John F. Kennedy’s grandson, Jack Schlossberg, 31, in which he employs a variety of accents to belittle his cousin as disingenuous and unintelligent.

On social media, Ms. Kennedy’s posts include clips of her father’s speeches, odes to human rights leaders such as John Lewis and tributes to her 96-year-old mother. (“Mummy is a masterclass in championing the rights of others,” she wrote recently.) Ms. Kennedy said she had not spoken to Mr. Schlossberg about his posts.

Ms. Kennedy now finds herself riding the crest of what is an unusual moment for a family that prides itself on keeping internal dissent out of the public eye.

“They’ve always learned from J.F.K.’s generation that you keep all the bad things that are inside the family inside the family,” said Laurence Leamer, who has written a series of books on the Kennedy family. “That is ending here.”

But through it all, the family has sought to stick together. “This is not against Bobby — this is for Biden,” said Kathleen Kennedy Townsend, an older sister. “We’ve been for Biden a long time.”

Kerry Kennedy, said she still saw her brother at family events, including at the Kennedy compound in Hyannis Port, Mass. “Bobby was up to the Cape a few times last summer,” she said. “We were both in Aspen on vacation and sitting around the lunch table. We send texts — we have a family text exchange — so there’s back and forth. But he’s not calling me for me advice.”

Christopher Kennedy said he frequently shared his concerns with his brother about what he was doing. “People confront me at the grocery store — ‘Why are you letting your brother get away with this?’” he said. “I think we will all get blamed if Bobby causes this outcome to occur.”

“I would say that, like all true believers, he is unreachable,” he said. “That hardening of the shell has definitely occurred.”

Ms. Kennedy said she was resigned to the fact that her brother was pushing on through the election and even suggested that she had come to accept — or at least understand — this latest campaign by a Kennedy.

“In the end, why is he out there doing this?” she said. “For the same reason I’m out there. Because we both believe the stakes are high.”