'Then she brought the baby to her people, carrying him. They said: "O Mary (Maryam)! Indeed you have brought a thing an unheard mighty thing. O sister of Aaron (Harun)! Your father was not a man who used to commit adultery, nor was your mother an unchaste woman.' 19:27-28
Islam critics often point out that the fact that Mary, mother of Jesus, is called the sister of Aaron (and of Moses, as Aaron was Moses' brother) is proof that the Qur'an can't be from God, as this is a mistake. Even Karen Armstrong mentions this 'mistake' in her book 'Muhammad'. Scholars have very plausible explanations for this though. When the Qur'an calls the mother of Jesus 'sister of Aaron' it isn't meant literally. It could simply refer to her noble lineage as in calling Jesus 'son of David'. There is also a tradition in the Arabic language in which family references like 'son/sister/brother of' simply mean 'from the tribe of'.
Another explanation is that Aaron's sister, who was also called Maryam, like Jesus' mother was known for her virtuousness and piety. By calling Jesus' mother 'sister of Aaron' it refers to the two Maryams having in common that they were both very virtuous and pious women. It is used in defending her against the accusations of unchasteness. I used to have a neighbour who often played football with his young son in the garden. Sometimes you could hear him shout 'come on Gary Lineker' to his son, ascribing to him the qualities of the great player. In a way this is not dissimilar to the way Aaron's sister is referred to in this Qur'an verse.