"I'm very, very deeply grateful to the governor for that decision," Kennedy said on Good Morning America.
Newsom vetoed the California Parole Board and said RFK's killer would not be granted parole, as he has shown no remorse for his crimes.
"I think this is a man who has shown no remorse, who has shown no ability to take responsibility for his crime, and he is a current danger to the people of California and to our country," Kennedy added.
Sirhan has been jailed for 53 years following his conviction for killing the former attorney general. However, not all of RFK's children share their sister's opinion regarding their father's murderer.
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Several have been outspoken regarding their desire to see Sirhan freed.
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"I come from a family with 11 children, and from my grandma and grandpa, Rose and Joe Kennedy, we have over 150 family members," Kennedy said. "So there's always gonna be somebody in there who doesn't agree with you, but I think that the thing to focus on here is who is this man?"
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Her father's assassination was planned for years, Kennedy said.
Sirhan not only purchased the bullet that was "most likely to be fatal," but he also went to target shooting practice the morning of the assassination, she added.
Her father was fatally shot, and five other people were injured by Sirhan's hand on June 5, 1968, at the Ambassador Hotel in Los Angeles.
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"This was not a simple killing. This was a massacre, and he nearly killed my mother, who was pregnant with my younger sister," according to Kennedy.
Sirhan's attorneys claim he serves no threat to society and must be freed, but the daughter of the man he killed said that idea is "just wrong."
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She mentioned a recent incident between Sirhan and an inmate in which he reportedly said he wanted to show the inmate "that I could hurt him very badly.'"
"This is somebody with a hair-trigger emotional reaction anytime he feels that he's been wronged, and it's easy to get a handgun in this country," Kennedy said. "He used a handgun smaller than the size of a fist to kill my father and shoot five other people, and he's shown none of these needed emotional — deep emotional change one has to go through in order to be released to society, in order to earn that release."
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