Every Christmas Kerry Kennedy makes a book for her numerous relatives. “It has at least one photograph of every single member of my extremely enormous family,” she says. “And yes, Bobby is in the book.”

“Bobby” refers to her brother, Robert Kennedy Jr, a hopeful sign that the sibling bond will survive an oncoming storm. Robert is running as an independent candidate for US president in November’s election. Kerry is one of at least 15 members of the Kennedy clan who recently endorsed Joe Biden instead.

Robert’s long history of promoting vaccine conspiracy theories, and associating with racists and antisemites, has been a source of anguish for what was once seen as America’s equivalent of a royal family. They have been at pains to distance themselves from the 70-year-old’s dangerously fringe, anti-scientific views.

Last year Jack Schlossberg, the sole grandson of former president John F Kennedy, denounced Robert for “trading in on Camelot, celebrity, conspiracy theories and conflict for personal gain and fame” and described the candidacy as an “embarrassment”.

With Robert polling at about 10% with potential to have an impact in crucial swing states, the Kennedy family has closed ranks around Biden, who keeps a bust of one-time presidential hopeful Robert Kennedy Sr in the Oval Office. Last month they joined the president at a campaign stop in Philadelphia to publicly back him against Donald Trump, whom they cast as a dire threat to American democracy.

Robert responded on social media that his family was “divided in our opinions but united in our love for each other”.

Kerry, 64, the seventh of Ethel and Robert Kennedy Sr’s 11 children, recalls in an interview that the collective decision involved “a lot of texts and emails and phone calls and ‘let’s do that’”.

Several notable members of the dynasty did not endorse, including Caroline Kennedy, the US ambassador to Australia, and the non-profit leader Maria Shriver, which the Biden campaign said was due to their non-political professional roles. Kerry adds: “Then there’s my brother Bobby and one cousin but 100% of everybody else endorsed Joe Biden.”

How did it feel to go against her own brother? After a pause, the human rights activist and lawyer says by phone from the family home in Hyannis Port, Massachusetts: “I feel like there’s so much at stake. When Daddy ran for president, in part of his speech he said, ‘I cannot stand aside,’ and that’s how I feel. I just feel there’s so much at stake.

“I’ve spent the last 40 years working on civil rights domestically and international human rights and trying to hold governments accountable for abusing people, particularly members of the press, and I can’t let this go. We cannot have Trump in for four more years. And Biden is great. He has accomplished so much more as president, which is wild considering the Congress that he’s had to work with.”

She points to historic legislation that Biden signed to tackle the climate crisis and slash child poverty, and to an economy that has made a better post-pandemic recovery than China, Europe or any other major competitor.

“Most Americans are doing a lot better than they were and Trump would be a disaster for us. I felt, and my siblings and my cousins all felt, that what’s at stake here is our democracy, our freedom. our fight for the middle class and all that means that Joe Biden must be re-elected.”

Robert, an environmental lawyer, has pushed bogus assertions about the dangers of vaccines, linked antidepressants to school shootings and claimed last year that the coronavirus was “targeted to attack Caucasians and Black people” and that “the people who are most immune are Ashkenazi Jews and Chinese” (he later said his remarks were misinterpreted).

But Kerry, president of the organisation Robert F Kennedy Human Rights, is reluctant to comment on her brother’s public statements or policy agenda, citing the impossibility of an independent candidate breaking Democrats and Republicans’ stranglehold on the electoral college.

“I don’t think that matters because he can’t win,” she says matter-of-factly. “It’s irrelevant what he says about any given subject. It’s a nonentity. He can’t get 270 electoral votes. The only question is not where he stands on a particular issue but what’s his impact on the campaign, and that to me is dangerous because this election, just like every other presidential, is going to be razor thin and we can’t afford to lose one vote – not one.

“We need every voter who’s thinking of voting for a third party to vote instead for Biden because otherwise it’s like you’re throwing a vote to Trump and that’s a disaster.”

Robert, initially challenged Biden in the party primary election before running as an independent, could use his last name’s lingering Democratic mystique to siphon support from the president. A super political action committee supporting his campaign produced a TV advert during the Super Bowl that relied heavily on imagery from John F Kennedy’s 1960 presidential run. (Robert apologised if the commercial “caused anyone in my family pain”.)

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But there is some evidence that he might hurt Trump more. A Quinnipiac survey last month found that Robert drew support from 12% of voters who supported Trump in a two-way contest, compared with 7% from Biden voters. An NBC poll found that Robert attracted 15% of Trump supporters compared with 7% of those of who backed Biden.

Kerry, formerly married to the Democrat Andrew Cuomo, an ex-governor of New York, responds: “That’s kind of irrelevant because I’m delighted for him to take every single vote he can from Trump but we want all those votes that would go to Biden to go to Biden because these margins are so close that we can’t afford to lose them to Bobby or anyone else.

Trump has been transparent about his ambitions for an imperial presidency. In a recent Time magazine interview, he said he was open to using the national guard to deport undocumented migrants and allowing states to monitor women’s pregnancies so they know if they receive an abortion.

Kerry offers a nightmarish vision of what a second Trump term would look like. He has said he’ll be dictator on day one, that he’ll suspend the constitution, that he will use the justice department to go after his enemies, that he will have a litmus test for the thousands of government workers on loyalty to Donald Trump and that loyalty test will be: was the election stolen or not? If you say the election was not stolen, you no longer have a government job.

“It would be generational bias on courts, not just the supreme court but courts across the board. He’s said he will create massive labour camps and forced detention centres for immigrants and he would put not just the police but the armed forces in our streets in order to enforce that.

“A Muslim ban on day one. It would be a disaster for global warming, as he’s indicated. It would be a disaster for women’s rights and women’s control over their bodies. You’d see more librarians going to prison for allowing books to be on bookshelves.”

At age 43, Kerry’s uncle was the youngest person ever elected to the presidency. Her father, a former attorney general, was just 42 when he was assassinated while running for the White House in 1968. There could be no greater contrast with Biden, who at 81 is the oldest president in history. Kerry, who has spent time with him twice in the past two months, offers her assessment.

“He physically does not have the grace of a 20-year-old but in terms of his mind, if you talk to him about any number of issues – and I’m not talking about what do you think about Gaza and the Middle East or Ukraine in general or poverty, I’m talking about what do you think about private prison systems that imprison Black youth in Louisiana? – he will talk to you about that.

“And then he’ll tell you about three different bills by name. ‘Well, HR 2732 would address that but the Republicans don’t want it for this reason and they have a congressman in Alabama who’s against it for this reason but there’s a chance of changing it on these two paragraphs.’ I mean, it is unbelievable. That is a guy who knows what’s going on. That is my experience.”

That is why, Kerry insists, voters must stay focused on the binary choice before them and not get distracted by third-party candidates: Jill Stein, Cornel West – and Robert Kennedy Jr. “I disagree with him on a range of the issues which I’ve discussed with him and been quite public about,” she says.

“But the point here is not a sister and a brother or a family or anything like that. It is what’s the future of our country and what’s the future of our world and do we have a democracy in the United States and do we have a liberal world order or not? That’s what’s going on here. Who cares about two siblings? It’s absurd.