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Heath Ledger‘s oldest sister Kate Ledger had an emotional journey watching the new documentary about his life, I Am Heath Ledger.

The film features rare recordings of the late actor before his untimely death in January 2008 — most of which Kate hadn’t seen before.

“He was incredibly private,” she explained to Australia’s The Project on Wednesday. “None of the footage that’s shown in the film actually came from the family. In fact, I didn’t even know it was in there until I saw it myself.”

“[It was] incredibly emotional,” Kate continued. “I think I screamed and cried through the whole first viewing. It’s quite a cathartic experience, especially for our family.”

I Am Heath Ledger arrives a little more than nine years after the actor’s tragic death at age 28 from an accidental mixture of prescription drugs. The first trailer was released on April 4 — which would have been Ledger’s 38th birthday.

On the eve of the documentary’s release, the actor’s family and friends spoke extensively with PEOPLE about his legacy — and the deep love he had for those in his life, including his daughter, Matilda.

“It’s almost like he had actually filmed the documentary himself and pieced it together for her,” Kate told PEOPLE.

“There was the movie star and then there was the person, and they just happened to inhabit the same body but they were not the same person,” his childhood friend Kane Manera shared with PEOPLE.

Hailed as one of the most compelling actors of his generation, Ledger was renowned for disappearing into his characters — including a tortured gay cowboy in 2005’s Brokeback Mountain, which earned him his first Oscar nomination, and his terrifying Joker in 2009’s The Dark Knight, which won him a posthumous Academy Award.

But off-screen, he was known for his rugged good looks and low-key demeanor — often seen skateboarding down the streets of his Brooklyn neighborhood and strolling with Matilda, the 11-year-old daughter he had with actress Michelle Williams.

The documentary, which premiered at the Tribeca Film Festival, explores Ledger’s personal side — as seen through his own eyes, with extraordinary home video the actor filmed of himself. It also recounts Ledger’s most memorable moments onscreen, from his beginnings in Australia to the height of his Hollywood celebrity.

Heath’s family gave I Am Heath Ledger filmmakers Adrian Buitenhuis and Derik Murray their blessing — and appeared in interviews for the documentary alongside those who knew Ledger best, like his ex Naomi Watts, Emile Hirsch, Ben Mendelssohn, Djimon Hounsou, Ang Lee, and other close friends and industry peers.

“We’re very happy with the way it turned out,” Kate told The Project.

One of the sweetest scenes in the film features a young Heath — and future Joker — dressed in a Batman T-shirt.

“That was on a cruise ship we went on as a family,” Kate recalled to The Project. “Mom knew there was going to be a dress-up night so she had organized costumes ahead of time. But yeah it was kind of a surreal experience, I suppose, for him to go on and be in a Batman movie.”

Since Heath’s death, Kate said she’s found it “hard” to revisit his films — like The Patriot and 10 Things I Hate About You. But she’s been able to recently watch them again through her children.

“I now have 13-year-old girls who want to see the films. So through them, it’s become a little easier for me to see the joy that it gives them,” Kate said.

And though Williams didn’t appear in I Am Heath Ledger, Kate said “she certainly gave it her blessing.”

“She’s an artist and I think she’s beautifully represented in the film,” Kate explained. “And that was her prerogative. I don’t think she needs to be in it — she’s represented. That was just a personal, private decision which we all 100 percent respect.”

I Am Heath Ledger is set to premiere on Spike TV on May 17.