Jack Nicholson was born to an unwed mother who said she wasn’t certain of his true father. He was raised by his maternal grandparents as if he was their child. Only learning that his “sister” was really his mother as a young man.
(Not So) Fun Fact: Jack Nicholson Thought His Mother Was His Sister
Neko Random 
A not uncommon deception back in the bad old days when unwed mothers were considered a shameful family secret.
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