In the Qur'aan, Hadhrat Maryam (alayhas salaam) was addressed by the people: "O sister of Haaroon!".

Answer

There are two versions. She was from the progeny of Nabi Haroon (alayhis salaam), hence this manner of address. That was the customary manner in addressing someone related to a great person. The other view is that Maryam (alayhis salaam) according to the Mufassireen had a half-brother, i.e. same father but different mothers. Her own mother did not have a son.