— -- The sister of Chris McCandless, the hiker whose two-year odyssey across America and into the Alaskan wilderness was immortalized in the bestselling book "Into the Wild," says his expedition was not just about his love of nature and his adventurous spirit, but also reflected his intent to sever ties with his parents after what she calls traumatic childhood.

McCandless was 22 years old and newly graduated from college in May 1990 when he set out across the American West in a trip that would ultimately take him to the wilds of Alaska.

His journey was immortalized in the book, which is still on school curriculums across the country, and later turned into an award-winning movie. The book detailed how after graduation McCandless suddenly disappeared, not telling his family where he was going, giving his savings to a charity, using the name "Alexander Supertramp," and then hitchhiking across deserts and the Great Plains, riding the rails and living in a trailer park on the rugged California coast before finally making his way to Alaska.

Just over four months after he reached Alaska, McCandless' body was found by hunters in an abandoned bus he had found 30 miles away from the nearest town. McCandless had died of starvation. His corpse weighed just 67 pounds.

According to his journal, for 114 days, Chris lived in what he called his "magic bus." By the end, he had written that "death looms" and he was "too weak to walk out." He wrote that he had "literally become trapped in the wild."

McCandless has been criticized for being selfish and unprepared for his trip. He was there without a map and proper survival gear, and he had gone to great lengths to make it impossible for anyone to find him. But today, every spring, hikers from around the world still make the two-day trek to the bus, which has become a shrine to the young man many idolize as a symbol of adventure and a turning away from material things.

McCandless' sister, Carine McCandless, and his half-sisters, Shelly and Shawna, say there is something else that many people don't know. They are sharing what they say is a vital part of their brother's story, one that better explains why he had gone to such great lengths to vanish from their view on his epic excursion.

"Frankly I was asked every time I met with a group of people... why Chris left the way he did and why he felt the need to push himself to such extremes," Carine told ABC News. "I really watered down those answers for a long time ... and I really felt and learned that I was doing a disservice to Chris and all those people 'cause the greatest inspiration comes from truth."

In a new memoir, "The Wild Truth," Carine writes that she believes her brother's sudden disappearance and journey reflected his determination to separate himself from their parents and a traumatic childhood that she says they both shared.

"He wanted to really separate himself from a situation he felt was very toxic," Carine told ABC News.

The whole truth, she said, doesn't begin at the bus in Alaska, but rather at their childhood home 3,000 miles away in El Segundo, California.

"He was Chris. He was my protector," she said. "He was always strong. He succeeded at everything he tried."

Carine said she has fond memories of family vacations and time spent outdoors with her family. Her father Walt McCandless is a renowned rocket scientist who had worked for NASA, and her mother Billie McCandless built a consulting business with him.

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