Is jack nicholson's sister his mother?

No, Jack Nicholson's sister is not his mother. Jack Nicholson was raised by his grandparents and believed his mother was his sister for much of his life. It was later revealed that his sister, June Nicholson, is actually his biological sister, while his mother, who was also named June, was his caregiver. This complex family dynamic has often been a topic of discussion in Nicholson's life and career.