In the summer of 1969, Hollywood suffered one of its worse horrors, and the loss of one of its brightest stars on the rise, Sharon Tate.
Her career had taken off, and a year earlier she married French-born film director Roman Polanski and was now eight months pregnant with their first child.
"Sharon was a beautiful person in every way," says Debra, 65, whose features uncannily echo those of her actress sister.
"She was kind and loving, and it would never have been in her to hurt anyone. Everyone who met her recognised the sweetness in her.
"This is why the story still haunts us. It is the good against the darkness and the evil, the sum of our fears."
"We were amazingly close as kids. Our dad was in the military, and we were always upping and moving home, so the only real constant Sharon and I had in our lives was each other.
"When she became a star she didn't change at all. Everyone knows that Hollywood can be a cruel place and, God knows, there's a lot of dishonesty and double-crossing, but Sharon never played that game.
"I've met so many people over the years who met her, maybe even just once, and remembered something nice she had said or done."
Just a year before her death, in 1968, she married 35-year-old Roman, whose dark cinematic eye was attributed to his having lost most of his Polish-Jewish family in the Holocaust.
"Sharon was completely crazy about Roman," says Debra.
"He was charming, smart, funny, brilliant. He still is. I speak to him on the phone quite often, and although he has moved on, as he had to, what happened back then is always with him.
"He loved her very deeply, but to this day he can't talk about it. He probably opens up to me more than anyone because we share the same sense of loss, but basically he can't go there."
On the weekend of August 9-10, 1969, with Roman delayed in London, Sharon invited friends over for dinner at their rented house on Cielo Drive.
None of them would survive what the Los Angeles prosecutor Vincent Bugliosi later characterised as "perhaps the most bizarre, savage, nightmarish murder spree in the recorded annals of crime."
Fifty years on, the killings, orchestrated by hippy cult leader Charles Manson and largely carried out by his female followers, have lost none of their power to terrify and astonish.
Manson, a would-be musician who thought he should be "bigger than the Beatles", saw the house and its occupants as symbolic of an aloof show-business establishment which had failed to recognise his genius.
"I get abused, threatened all the time," tells Debra. "There are a lot of people out there who still venerate Charlie, who want the Family out of jail, so I have to keep fighting. Otherwise Charlie gets what he wanted, which was to have recognition, to be a superstar."
Although her health is shaky and she receives regular death threats from the dark extremities of the Manson-worshipping web, Debra campaigns relentlessly against the release of the imprisoned members of the Family.
"We are fooling ourselves if we think these people are no longer dangerous. When has any of them shown real regret? Why do they keep in such close touch with each other? I hear their lawyers saying they are reformed and shouldn't still be paying for their youthful mistakes, and I think, bull."
WATCH BELOW: The trailer for Once Upon A Time In Hollywood. Story continues after video.
Debra has long been sceptical of the "brainwashing" argument peddled by defence lawyers.
"This is what I have to keep fighting against," she says. "The idea that Charlie so controlled them that they didn't know what they were doing is absurd. It's impossible. If it's true, why did some of them refuse to take part? He chose the ones he knew had it in them, and when he said, 'Will you kill for me?' they willingly said yes.
"These people are sociopaths. They knew what they were doing. I've read all the psychological reports. They belong in jail."
Many of the lingering questions about the case will be aired in the crop of new films marking the anniversary – notably Quentin Tarrantino's star-laden Once Upon a Time in Hollywood, with Brad Pitt, Leonardo DiCaprio, and Australian actress Margot Robbie as Sharon.
"Tarantino came to see me and he was extremely respectful," she says.
"He listened to my point of view that the victims' families should be treated with consideration, and he let me see the script, and we spent a long time talking about the whole project."
"I do think he's handled things sensitively," Debra says. "I also think Margot Robbie is perfect as Sharon."
Once Upon a Time in Hollywood hits Australian cinemas on August 15.
Read William Langley's full interview with Debra Tate in the August issue of The Australian Women's Weekly, on sale now.