Who did jack nicholson think was his sister?

Jack Nicholson thought June Nicholson was his sister for many years.

The revelation that June was actually his mother, and their mother Ethel May was actually his grandmother, came as a shock to both of them when it was uncovered in 1974. It was a complex family situation where Ethel May had raised June as her own daughter, and never told Jack about the true nature of their relationship.