Who did jack nicholson think was his sister?

Answer 1

Nicholson, who grew up in New Jersey, was told that his mother, June, was his sister. She was only 16 years old when he was born in 1937, so her mother reared the boy so June could continue to work as a dancer. Nicholson did not learn the truth until he was 37 years old, after the deaths of his mother and grandmother.

Answer 2

He was bought up believing that his biological grandparents were actually his parents and his biological mother acted as his sister. So, no, his sister isn't biologically his mother at all he was just lied to as a child that she was his sister and later on he found out she was actually his mother.