Jack Nicholson discovered sister was actually his mother after they had both died

Legendary actor Jack Nicholson turns 89 today but the retired film star also went through an M Night Shyamalan twist in his own personal life.

The American was born in New Jersey back in 1937 but the identity of his real mother was kept hidden from him until he turned 37, as he was informed that the woman he believed to be his sister was actually his biological mother, and the woman he had called mum was actually his grandma.

The Oscar-winner only found out after both of them had passed away, with it seemingly done to protect the family from scandal as his biological mother June was just 18 when she became pregnant with a married man's child.

In order to avoid the controversy that came with June and Don having a child together, in what was a much more conservative time, his grandmother Ethel May raised him as if he was her own son.

He ended up following June to Los Angeles when he was just 17, so we perhaps have the baffling family arrangement to thank for his impressive career, which has seen him star in classic films such as The Shining and One Flew Over The Cuckoo's Nest.

Nicholson won his Best Actor award in 1976 (Kevork Djansezian/Getty Images)

Nicholson told Rolling Stone in 1986 that he joined his biological mother in LA, unaware of their true relationship, saying: “Since my only relative in the world was June, who was out here, I came out to look around.”

After working as an assistant in the animation department at MGM Studios, he eventually started making his name as an actor and it was actually ahead of his film Chinatown in 1974 that the truth finally came out, following an investigation by journalists at TIME magazine.

No doubt interested in learning more about one of the new kids on the block, the journalists decided to do some digging ahead of a feature piece and quickly discovered that not everything was as it seemed, even revealing that his father may have still been alive at the time and living in New Jersey.

Jack's son Ray has also entered the world of acting (Kevork Djansezian/Getty Images)

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

His Aunt Lorraine, who he had believed to be another sister, later confirmed that it was true following the deaths of both Ethel and June, although he also later admitted that he didn't feel any sort of resentment towards either woman when it came to taking their secret to the grave.

He said: “I was very impressed by their ability to keep the secret, if nothing else.

“It’s done great things for me.”

So, should we ever get a biopic about one of the best actors from recent times, then now you know to expect a pretty big real-life twist.