While most of the world knew Christopher Reeve as Superman, to his three children – Matthew, Alexandra and Will Reeve – he was simply their beloved father.

The three siblings watched firsthand as their father rose from movie star to pioneering activist for spinal cord research after a horse riding accident nearly killed him in 1995. At age 42, he was paralyzed from the neck down.

Twenty years after Christopher Reeve’s death in 2004 from heart failure, his life, work and legacy are explored in a new documentary, “Super/Man: The Christopher Reeve Story,” which features all three of his children.

The Reeve siblings spoke with ABC News’ Diane Sawyer about their memories of their father and his wife Dana Reeve, who died of lung cancer in 2006, just two years after her husband’s death.

“I think it doesn’t matter what a family looks like, no matter how shaky the branches of the family tree are, as long as the roots are in love, it doesn’t matter what your family looks like, as long as it’s yours,” Will Reeve said of his family’s recovery from tragedy.

Will Reeve, now an ABC News correspondent, was only 2 years old when his father had a horse riding accident. His half-siblings, Alexandra Reeve and Matthew Reeve, were 11 and 15, respectively.

The siblings remember the first moments they saw their father after the accident.

“Dana walked with us and said, ‘It looks really scary, but he’s still there. Just talk to him. Ignore everything. Just talk to him,'” Alexandra Reeve recalled, adding, “She said, ‘You can hold his hand.’ And there’s machines on it, you know, and you reach for his fingers and hold them.”

Alexandra Reeve, now a lawyer, remembers holding her father’s hand as so many machines worked to keep him alive.

Matthew Reeve recalled that when he saw his father for the first time after the accident, he simply told him, “I love you. We’re here.”

“I think my mom (Dana Reeve) made sure that I was involved in a way that was reasonable, to make sure that I wasn’t permanently afraid of my dad,” Will Reeve added. “And that included me always being around him, touching him, helping to lift his legs up so that I could see him, which is important for someone with a spinal cord injury.”

When Christopher Reeve came home from the hospital, the siblings said that watching Christopher and Dana Reeve taught them that life goes on. And while life was different, the love within the family was the same.

Will Reeve said he has memories of a “happy” and “loud” home full of family dinners and joy.

“Whether it was good news, bad news or scary news, dinner was family time,” Will Reeve said. “My mom cooked, (and mom and dad) sat at the head of the bed, next to each other. My mom fed herself and him.”

He continued: “We had friends coming over. It was a very cheerful, robust, noisy, everything you would expect from a family dinner, and that was every night. And the only thing you weren’t allowed to talk about was specific medical stuff. It could be anything.”

Matthew Reeve remembers a house “full of music,” thanks to Dana Reeve, who was a professional singer when she met her future husband at age 25.

“She would always burst into song, and that’s one of the things I miss most,” said Matthew Reeve.

Also reflecting on how his mother “always sang”, Will Reeve added: “It’s easy to say in hindsight, but I wish I had asked her to sing more, because when I think of her, that’s where I go … the quiet moments of safety and happiness and normality and togetherness. We didn’t even have to be in the same room, but I knew where she was. I miss that.”

Like other children, the siblings looked for ways to make their father laugh.

“Matthew came back to college with a gift for his dad, and it was an eject button to put on the hand of his wheelchair, right by his finger, and dad thought that was awesome,” Alexandra Reeve recalled. “Someone’s in a serious meeting with Christopher Reeve, and they look at his wheelchair, and they just see this eject button, and you can see it register on their face, like, ‘Ask? Don’t ask? What do we do?’”

In the decade following his accident, Christopher Reeve inspired not only his family but millions of people around the world as an activist and advocate for spinal cord injury research, through the Christopher & Dana Reeve Foundation with his wife.

According to the foundation’s website, the foundation has now invested more than $140 million in research.

Remembering the Legacy of Christopher and Dana Reeve

Less than a year after delivering a eulogy at her husband’s funeral, Dana Reeve, a lifelong non-smoker, was diagnosed with stage 4 lung cancer.

Seven months later, on March 6, 2006, she died at the age of 44.

Will Reeve says in the documentary that it was only after his mother’s death that he read her diaries and fully understood the difficulties she faced as a caregiver for her husband and family.

“I’ve been studying the difference between loneliness and solitude. I tell my life story to the clean white towels I pull warm from the dryer and hold to my chest, a sad substitute for a body drawn toward me. I think of him in a certain light. Dawn, late afternoon, bright, windy days that would be perfect for sailing,” Will Reeve reads from his mother’s diary in “Super/Man.” “What I still miss most are his hands. Their expressive grace and weight. The warmth of his hands on my skin. The folds of his arms, two becoming one. I carry the stack of towels upstairs, holding them carefully so as not to drop them. Except for one that’s still damp, the top one I’d pressed to my face, which needs more time to dry.”

“Super/Man: The Christopher Reeve Story” premiered at the Sundance Film Festival in January. The film is scheduled for a limited release in U.S. theaters on September 21 and 25, with tickets available through Fathom Events.

ABC News’ Carson Blackwelder contributed to this report.