Debra Tate grew up Catholic, which means that she has somehow forgiven the people who strung up her heavily pregnant sister, stabbed her 16 times and daubed foul messages with her blood on the walls. But that does not mean she has forgotten.

The 66-year-old has devoted most of her adult life to campaigning to keep Charles Manson's acolytes - who massacred Sharon Tate, her unborn child and four others on August 9 1969 - behind bars. The events of that night 50 years ago have inspired countless column inches, books and films; now another, Quentin Tarantino's Once Upon a Time in Hollywood, which is released in New Zealand this Thursday.

Debra was just 16 at the time, but her memories are vivid. Sharon, 26, had phoned home on the day she died, and asked to speak to her little sister. "It was a blisteringly hot day," she recalls. "I was supposed to go to Sharon's, then she called and said 'I don't feel like moving, it's so hot'." That was the last time she heard her voice.

Instead, Sharon went out for dinner with friends Jay Sebring, Wojciech Frykowski and Abigail Folger that night, the group returning at around 10pm to the luxury rented home on Cielo Drive, just north of Beverly Hills, that she shared with husband Roman Polanski.

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A few hours later, four members of the Manson Family cult drove to the property, cut the phone line and shot an 18-year-old boy who had been visiting the groundsman, before breaking in and repeatedly stabbing and shooting everyone inside.

The so-called Tate murders, and those of husband and wife Leno and Rosemary LaBianca the next night, were seen as marking the definitive end of Sixties optimism, with rumours of satanic rituals and sex games gone wrong. In the aftermath, prosecutors claimed Manson ordered the killings to trigger a race war, intending to frame black Americans for the crimes.

As for Debra, she was in the shower the next morning, when "all of a sudden the door was flung open and there I am in all my nakedness with a neighbour and my mother in the bathroom. She said, 'Sharon's dead'.

"I wrapped a towel around my body, I didn't even dry anything. There were probably little wet footprints going all the way down the hall to the kitchen. Mum was having a breakdown."

Debra Tate, sister to actress Sharon Tate. Photo / Getty Images
Debra Tate, sister to actress Sharon Tate. Photo / Getty Images

The Tate family spent years trying to get over the tragedy of losing "lovely, kind" Sharon. Debra tries not to think about what her sister might have gone on to do: she was being groomed for stardom with several big roles already under her belt, and just a few weeks away from becoming a mother.

The shock was particularly hard on the sisters' own mother, Doris, who Debra says was "dysfunctional" for a decade afterwards. "I'd describe it as the lights were on but nobody was home."

Debra was initially worried about Tarantino's handling of her sister's murder, given his gory back catalogue. "You put Quentin Tarantino together with the Manson story and of course your imagination goes crazy," she says.

Margot Robbie as Sharon Tate in Once Upon A Time In Hollywood. Photo / supplied
Margot Robbie as Sharon Tate in Once Upon A Time In Hollywood. Photo / supplied

But her fears were assuaged when the director approached her. "He came to me and showed me the script that he had written, we took three days to discuss things," she says. The finished product, which she saw at its Hollywood premiere in July, is so good that she "didn't want it to stop".

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Margot Robbie, who plays Sharon, also visited Debra to find out what her sister was like. "She really did do her homework, she came to me with lots of lots of knowledge six weeks prior to [the film] being made," Debra says. "I gave her a bottle of Sharon's perfume and I lent her Sharon's jewellery to actually wear during a few of the scenes: earrings and rings."

Margot Robbie as Sharon Tate in Quentin Tarantino's Once Upon A Time in Hollywood. Photo / supplied
Margot Robbie as Sharon Tate in Quentin Tarantino's Once Upon A Time in Hollywood. Photo / supplied

Debra was not the only one concerned about how the murders would come across on screen: Polanski, who was filming in London at the time of the attacks, was worried too, though she believes that he is "OK" with how the film turned out.

He and Sharon, who met when he directed her in 1967's The Fearless Vampire Killers, were very much in love, says Debra. She's still in touch with him, despite the fact that he fled the US in 1978, before he could be sentenced for pleading guilty to drugging and raping a 13-year-old.

A dual French and Polish citizen, Polanski's behaviour was influenced by the fact that "in France that's kind of a normal thing: older woman, younger man, older man, younger woman," Debra maintains. "He never said he didn't do it, unlike Harvey Weinstein."

As for Manson, though he terrified the world with his black eyes and wild hair, he cut a cowardly, isolated figure in the flesh. She remembers him hiding from her when she attended one of his prison parole meetings. "The guard was undoing one of the two contraptions restraining him and he said 'Who's that?'. The guard said 'That's Sharon's sister'. He said 'Put my leg irons back on and take me back to my cell', and he never came back into another hearing."

Manson himself died, aged 83, in 2017, but she believes the five living killers still in jail (another, Steve Grogan, was released in 1985) are just as dangerous as ever, having never shown any remorse. "Nothing has changed: they have all of the bells and whistles of being narcissistic and psychopathic," she says.

Those inspired by the Manson myth menace her to this day. On one occasion, she returned to her gated property to find "a guy with a truck on the inside of the fence and he would not tell me how he got in".

"He had a clipboard in his hand and he wanted to lure me closer by looking at it. He had a box truck with no windows, so I kept my distance." Luckily, she had friends in the back of her car with her. "I yelled 'get the f--- off my property now or I'm going to call the police'," she says. "He got out of there quicker than anything else, [but] if I had been by myself I could have been in trouble."

Even more of a worry to her are the bubbling racial tensions in America, which flared up again with horrific consequences, last weekend, when 20 people were killed in an apparently racially motivated shooting at a Walmart in El Paso, Texas.

It seems plausible to Debra that a group like Manson's could spring up again. "There are still racists," she says. "It's not just the Manson Family, they're all sharing a common denominator and that is a hate... It's very likely [to happen again]."

Despite the subject matter, Once Upon a Time in Hollywood has given her odd moments of hope. At the premiere, "a little butterfly came and landed on a lantern right at my table," she says. "It stayed there for the longest time, people were pointing at it. I don't know who it was... most likely Sharon."