While most of the world knew Christopher Reeve as Superman, to his three children -- Matthew, Alexandra and Will Reeve -- he was simply their beloved dad, one who was gone from them far too soon. The three siblings watched firsthand as their father went from movie star to pioneering activist for spinal cord injury research after a near-fatal horse riding accident in 1995 left him paralyzed from the neck down at the age of 42. Then, in 2004, Christopher Reeve died unexpectedly due to heart failure. In addition to his children, by the actor’s side from his accident to his death was his beloved wife Dana Reeve, mom to Will Reeve and stepmom to Matthew Reeve and Alexandra Reeve. Less than one year after delivering a eulogy at her husband's funeral, Dana Reeve, a non-smoker her entire, was diagnosed with Stage 4 lung cancer. She died seven months later on March 6, 2006, at the age of 44. "Despite the love and security that my siblings provided me, and my family provided me, and my adoptive family provides me, that was the moment, March 6, 2006 ... I've been alone since then," Will Reeve, who was 13 when he lost his mother, said in a new documentary, "Super/Man: The Christopher Reeve Story,” that explores the actor’s life. Prior to her death, Dana Reeve made sure Will Reeve was taken care of, arranging for him to live with the family of his best friend. His older siblings also dropped everything to help him. Alexandra Reeve was a law student at the time and Matthew Reeve a producer. In their conversation with ABC News' Diane Sawyer, Will Reeve, now an ABC News correspondent, posed a question to his siblings that he had never before asked them -- did people worry enough about them after their father and Dana Reeve died. "I don't think I've ever thought about that either, like, that wasn't the job at hand," Alexandra Reeve replied. "The job at hand was keeping things going, keeping us OK, keeping everyone OK, honoring them in the right way, setting you up for success." Speaking to his brother, Matthew Reeve added, "I think our greatest focus and frame of mind was you." In "Super/Man," Will Reeve said it was only after his mother's death that he read her journals and fully understood the difficulties she faced as a caregiver for her husband and family. “I have been studying the difference between solitude and loneliness. Telling the story of my life to the clean white towels taken warm from the dryer and held to my chest. A sad substitute for a body pulled in close. I think of him in certain lights. Dawn, late afternoon, bright windy days that would be perfect for sailing,” Will Reeve reads from his mom’s journal in “Super/Man.” “I miss most even now his hands. The expressive grace and heft of them. The heat of his hands on my skin. The wrap of his arms, two becoming one. I carry the stack of towels upstairs carefully cradling them so as not to let them tumble. Save one still damp, the top one I had pressed against my face, which needs more time for drying.” All three siblings credit Dana Reeve with successfully guiding them through the “new normal” they experienced after Christopher Reeve’s accident. "I think no matter the contours of a family, no matter how scraggly the branches on the family tree might be, as long as it's rooted in love, it doesn't matter what your family looks like as long as it's yours," Will Reeve said of his family's endurance through tragedy. Will Reeve was just 2 years old at the time of his dad's horse riding accident. His half-siblings, Alexandra Reeve and Matthew Reeve, were 11 years old and 15 years old, respectively, at the time. The siblings remember the first moments seeing their father after the accident. "Dana walked us down and she said, 'It's gonna look really scary, but he's still there. Just talk to him. Ignore everything else. Just talk to him,'" Alexandra Reeve recalled, adding, "She said, 'You can hold his hand.' And it has machines on it, you know, and you're reaching and holding his fingers." Alexandra Reeve remembers holding her dad's hand amid all the machines working to keep him alive. Matthew Reeve recalled that when he first saw his dad after the accident, he told him simply, "I love you. We're here." "I think in an effort to make sure that I wasn't terrified of my dad permanently, my mom [Dana Reeve] made sure that I was involved as was reasonable," Will Reeve added. "And that included always being near him and touching him and helping lift his legs to range him, which is important for someone with a spinal cord injury." When Christopher Reeve came from the hospital, the siblings said they learned by observing Christopher and Dana Reeve that life would go on, and while life was different, the family love was the same. "There were days where he was getting major setbacks, devastating medical news, you know, or changes on a policy fight that he was fighting and he'd lost that battle, and he would let us see the hope and the disappointment and say, 'Today's a really hard day,' and then he would say, 'And we're gonna go get dinner together,' or, 'Let's go watch a movie,'" Alexandra Reeve recalled. "He would let us see him take that journey back up." Like other kids, growing up, the siblings said they found ways to make their dad laugh. "Matthew at one point in college came back with a gift to dad, and it was an eject button to put on the hand of his wheelchair, right by his finger, and Dad loved that," Alexandra Reeve recalled. "Someone's there in a serious meeting with Christopher Reeve, and they suddenly look at his wheelchair. They're just seeing this eject button, and you kind of see it register on their face, like, 'To ask? To not ask? What do we do?'" Will Reeve said his memories are of a "happy" and "loud" home full of family dinners and joy. "Whether it was good news, bad news, scary news, dinner was family time," Will Reeve said. "My mom would cook, [and mom and dad] sat at the head, next to each other. My mom would feed him and herself." He continued, "We had friends dropping by. It was a very happy, robust, loud, everything you would want from a family dinner, and that was every night. And the one thing you weren't allowed to talk about was specific medical stuff. Could be anything else." Matthew Reeve recalled a house "full of music," thanks to Dana Reeve, who was a professional singer when she met her future husband at age 25. "She was always just bursting into song, and it was one of the things I miss most for sure," Matthew Reeve said. Also reflecting on how his mom was "always singing," Will Reeve added, "Hindsight is 20/20, but I wish that I had asked her to sing more, because when I think about her, that's where I go ... the quiet moments of safety and happiness and normalcy and togetherness. We didn't even have to be in the same room, but I knew where she was. I miss that." Today, Will Reeve said he frequently sings to his siblings' children the same lullaby his mom always sang to him as a child, saying, "I remember the words. How could you not." "There's a universal story in here, and it's not about a famous person in a cape and tights. It's about a family," Reeve said of his family's story, told in "Super/Man." "It's a human story. We had human parents who did superhuman things." In the decade after his accident, Christopher Reeve not only inspired his family but millions of people around the world as an activist and advocate for spinal cord injury research, launching the Christopher & Dana Reeve Foundation with his wife. The foundation has since invested more than $140 million to date in research investments, according to its website. "Super/Man: The Christopher Reeve Story" premiered at the Sundance Film Festival in January. It is set for a limited theatrical release in U.S. theaters Sept. 21 and 25, with tickets available via Fathom Events. ABC News' Carson Blackwelder contributed to this report. © 2024 ABC News