It was in August 1982 when the first 18 years of Alex Lewis’ life vanished in a flash. The recent high school graduate was riding his motorcycle home from a friend’s wedding in West Sussex, England, when his bike spun out of control.

Alex lost his helmet in the crash and suffered severe head injuries that left him in a coma and fighting for his life. And when he woke up three weeks later, Alex’s entire identity and memory was erased. The only thing Alex knew was that it was his identical twin brother Marcus who was sitting by his side.

“He actually said, ‘Hello, Marcie,’ our nickname for each other,” Marcus, now 55, recalls in this week’s issue of PEOPLE. “Then the doctors started questioning him — ‘Do you know what day it is? Do you know your name?’ — and he didn’t know anything at all.”

After Marcus taught Alex how to relearn everything from how to walk to how to brush his teeth, the questions became more complex. Alex wanted to everything about their past, so Marcus helped fill in the gaps by sharing stories of their childhood with loving, quirky parents and fun-filled family vacations.

Video: ‘Tell Me Who I Am’ Official Trailer

The only problem: Nothing Marcus had told him was true. The reality was that the boys had a horrifying childhood, with a harsh and neglectful stepfather and a mother who had sexually abused them.

When Alex did find out the truth over a decade later, he felt betrayed by the one person he could trust.

“My brother had deceived me. All of a sudden I just couldn’t believe anything anymore,” says Alex, who, alongside Marcus, shares his story in a new Netflix documentary called Tell Me Who I Am, based on the brothers’ 2013 memoir of the same name.

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View photos Alex and Marcus Lewis with their mother Jill Dudley | Netflix More

Their parents’ deaths led to their secrets being revealed. After their stepfather, Jack Dudley, died from cancer in 1990 and their mother, Jill Dudley, from a brain tumor five years later, Alex was puzzled by Marcus’ lack of grief.

“His reaction was so different than mine,” says Alex.

By that time he had been seeing a therapist who had suggested, based on her observations, that there may have been abuse in the family, but Alex hadn’t believed her.

“I was horrified,” he recalls. “I said, ‘How dare you say that?’”

But as the twins were going through their mother’s things, they found a photograph of themselves at age 10, hidden in a drawer.

For more on Marcus and Alex Lewis’ story in the Netflix documentary Tell Me Who I Am, subscribe now to PEOPLE or pick up this week’s issue, on newsstands now.

They were naked, and the top of the photo, where their faces had been, was cut off. Suddenly something in Alex’s mind clicked.

“I asked Marcus if we were abused by Mummy, and he just nodded yes,” he says. “And that is a moment that he and I will never forget. From then on everything changed.”

For the next 20 years, as the brothers moved to London and started a property-development business together, Marcus never discussed the abuse.

“It’s not that I wouldn’t tell him,” says Marcus, who along with Alex are both married with children and live just 26 miles apart. “I couldn’t. I wasn’t capable of telling him. It was too dramatic for me.”

Then, in 2013, after Alex shared his story in an article for a London newspaper, the two were offered a book deal, which moved both brothers to open up to each other. That led to the Netflix production — a cathartic experience that helped the twins finally put their demons to rest.