Jack Nicholson was born on April 22, 1937, in Neptune, New Jersey, according to Biography, and grew up in the Jersey Shore city of Manasquan. His parents — at least, the two people who he thought were his parents — were John and Ethel May Nicholson. The young lad grew up with his “parents” and with June, 17 years his senior, whom he believed was his much older sister, and another “sister” (actually his aunt), Lorraine.
As it turns out, however, June had gotten pregnant and wasn’t sure who the father was. These days, teen pregnancy is handled with a bit more care and is discussed a bit more openly than it was nine decades ago, and indeed, Jack’s grandparents decided to raise the boy as if he were their own son, not their grandson, and never tell him the truth. June and Lorraine agreed, and Jack grew up with his mother and aunt acting the part of his sisters.
According to Showbiz Cheat Sheet, the family eventually moved to New York City and the townsfolk promptly forgot about any rumors.