It’s well-known the Kennedys can be ruthless — but even to their own?

According to a new book, at least one Kennedy threatened to blackmail JFK Jr. if he refused to support a cousin accused of rape.

In “RFK, Jr.: Robert F. Kennedy Jr. and the Dark Side of the Dream,” author Jerry Oppenheimer writes that John Jr. was forced to attend the trial of William Kennedy Smith — which generated worldwide headlines and 24/7 broadcast news coverage — against his mother Jackie’s wishes.

On March 30, 1991 — Good Friday — Smith and uncle Ted Kennedy picked up two young women during a long night of bar hopping in Palm Beach, Fla.



All four, along with Ted’s son, Patrick, returned to the Kennedys’ beachfront mansion, and one of the women, Patricia Bowman, later accused Kennedy Smith of raping her.

Three other women gave sworn statements that Kennedy Smith had assaulted them back in the 1980s, but the judge disallowed those.

Oppenheimer writes about another, lesser-known sworn statement — this one given to a member of Congress in 1993 by James Ridgway de Szigethy, an acquaintance of JFK Jr.

According to de Szigethy, Kennedy Smith’s behavior was a well-known Kennedy family secret, and John had told him that “ ‘they should have done something about Willie years ago when he first started doing this,’ meaning . . . raping women.”

JFK Jr. — then a Manhattan assistant district attorney — also confided “that when the [Smith] trial took place, he would have to put in an appearance in the courtroom. He told me he did not want to do this and his mother did not want him to, either. I suggested that he not do it since Willie was guilty, but he told me who was pressuring and why.”

De Szigethy stated that he argued with John over his attendance, which would only support the defense’s assertions that Kennedy Smith’s accuser was mentally unstable and sexually promiscuous.

You just don’t understand the pressure I’m under - JFK Jr. allegedly said

“I remember every word of the next two angry questions I asked him,” De Szigethy said. “ ‘How does it feel to be a character assassin, John? How does it feel to be Patricia Bowman’s Oswald?’”

JFK Jr. sat in silence, head down, for a long time. He told de Szigethy that if he didn’t show up, at least one member of his family would leak damaging information about him to the press.

The word “blackmail” was used in de Szigethy’s affidavit.

“You just don’t understand the pressure I’m under,” JFK Jr. said.

The Kennedy family’s crown prince attended twice, during crucial jury selection.

Kennedy Smith was acquitted on Dec. 12, 1991, after a 10-day trial. He has since been accused of sexual assault by two former employees; one complaint was dismissed in 2004, another was settled out of court in 2005.