Snorri Sturalsson says in places that she is the daughter of Olaf, rather than the sister of Olaf. Other Sagas say she is the daughter. Combine this with the fact that it is chronologically improbable that she was his daughter we can see Snorri was mistaken.

Kilde:
http://www3.dcs.hull.ac.uk/cgi-bin/gedlkup/n=royal?royal18722

Married ABT 996 to Hakonson, Svein, Earl of Ladir, Norway

Child 1: Sveinsdottir, Sigrid
Child 2: Sveinsdottir, Gunnhild