Jack Nicholson found out his sister was actually his mother aged 37 after they had both died

Jack Nicholson once found out that his sister was really his mother, after both of them had passed away.

The shocking revelation wouldn't look out of place in one of the actor's many dramatic movie titles, with the 88-year-old being the subject of his own twist in the script.

Nicholson was born in 1937 in Neptune City, New Jersey, but it turned out that his upbringing was built on a lie, seemingly to protect the family from scandal.

The Oscar-winning star thought that the woman who raised him was his mother, but it turned out that she was really his grandmother, with the woman who he believed was his sister, being his biological mum.

He believed this for almost 40 years, before finding out about the arrangement at the age of 37.

Jack Nicholson is one of the biggest Hollywood actors in history (Evening Standard/Hulton Archive/Getty Images)

His biological mother, June Nicholson, was 18 when she became pregnant, but his father, a man named Don, was already married - so to avoid any trouble in the much more conservative 1930s, his grandmother, Ethel May, raised him as her own.

June was later introduced to Nicholson as his far older sibling, while her sister Lorraine was also introduced as his sister (she was actually his aunt).

The former would leave for Miami to pursue a career as a showgirl, while Nicholson would be raised by his biological grandmother.

Nicholson told Rolling Stone in 1986 that he followed June to Los Angeles at the age of 17, unaware of their real relationship, recalling: “Since my only relative in the world was June, who was out here, I came out to look around.”

He was an office assistant in the animation department at MGM Studios before making his name as an actor, but in 1974, prior to the release of Nicholson's film Chinatown, his family's truth came out.

Journalists at Time magazine were researching for a feature profile, uncovering the shocking truth about the Nicholsons, even finding that his father may still be alive in New Jersey.

The actor reportedly said at the time: “This is the most f***ed thing I’ve ever heard,” biographer Patrick McGilligan claimed.

He had no ill-feelings towards his family after finding out (Getty Images/Bob Riha, Jr.)

Nicholson then called a close relative for answers and while they denied the claims at first, Lorraine revealed the truth, as both June and Ethel May had passed away at this point.

It meant that The Shining star couldn't confront either woman about the revelation, though he did say that he felt no resentment towards his family, telling Rolling Stone: “I was very impressed by their ability to keep the secret, if nothing else.

“It’s done great things for me.”

Nicholson has joked about his 'sister-mother' in the past, but he doesn't really address the revelation publicly, in what might be one of the industry's biggest real-life twists.