The Shrivers’ broadside was the latest in a series of attacks on Mr Kennedy Jr by his family.
In October, his four siblings publicly voiced their opposition to his presidential campaign, which they said was “deeply saddening” and “perilous for our country.”
They have been infuriated by Mr Kennedy’s incendiary remarks about vaccines – such as when he claimed those who opposed the Covid vaccine were being persecuted more severely than Ann Frank, who died in a Nazi concentration camp.
And in December 2021, he defied his mother Ethel Kennedy by supporting the release of Sirhan Sirhan, who was convicted of texting his father Bobby Kennedy in June 1968 when he was the favourite to win the presidential nomination.
Fears RFK Jr will hurt Biden more than Trump
Mr Kennedy’s supporters have sought to promote him as holding the same values as his uncle and father, often leaning on his famous family name and celebrity connections.
The message was hammered home again by Tony Lyons, the co-founder of the American Values Super PAC.
Mr Lyons said in a statement on Sunday: “RFK Jr offers us real change along with freedom, trust and hope. Like his uncle and his father, Kennedy is a corruption-fighter, and it’s no wonder the DNC is trying every old trick and inventing new tricks to stop him. The public sees through it all and won’t stand for it.”
And despite being a rank outsider, Mr Kennedy’s poll numbers are better than those of Joe Biden and Donald Trump. According to FiveThirtyEight, he has a positive “net favorability” rating of 7.4 per cent
There are growing fears his entry into the race will hit Joe Biden and help Donald Trump.
Democrats have also attacked his decision to run, pointing out that Timothy Mellon, a major donor to his Super PAC, has also given money to the Trump Super PAC.
“It’s fitting that the first national ad promoting Robert F. Kennedy Jr.’s candidacy was bought and paid for by Donald Trump’s largest donor this cycle. RFK Jr. is nothing more than a Trump stalking horse in this race,” said Democratic National Committee spokesman Alex Floyd.