For decades, Caroline Kennedy kept silent.

Ms. Kennedy, 67, the only surviving child of President John F. Kennedy, has been a model of two of the family’s prized values: discretion and stoicism. Handed no shortage of tragedy and scandal, she has kept quiet and largely stayed on the sidelines.

In recent years, that has meant that Ms. Kennedy has said little as her cousin Robert F. Kennedy Jr. spread false health information during a pandemic, challenged a Democrat for president, endorsed Donald J. Trump for the White House and then lobbied for a place in his cabinet. She was largely silent as he used his family’s name — and the likeness and memory of her father — to advance his campaign while, in the view of many of the Kennedys, defying what the family has stood for across generations.

The closest the understated Ms. Kennedy came to criticism was to note, from her diplomatic post in Australia, that Mr. Kennedy’s views on vaccinations were “dangerous.”

But on Tuesday, Ms. Kennedy, 67, unleashed a searing public denunciation of her cousin, delivering a moment that stood out even after the family’s 60 years of public triumphs and tragedies.

On the eve of his Senate confirmation hearing, Ms. Kennedy described Mr. Kennedy as not just unqualified to run the Department of Health and Human Services, the position that President Trump has nominated him to fill, but also a “predator” and a hypocrite.

She not only wrote this in a letter to senators; she also posted a video of her reading it, coolly and methodically. The fact that the public rarely sees her speaking made it all the more devastating.

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