19.27: And she came to her people with him, carrying him (with her). They said: O Marium! surely you have done a strange thing.
19.28: O sister of Haroun! your father was not a bad man, nor, was your mother an unchaste woman.
When Quran uses the term “brother” or “sister,” it is not using it literally. Examples abound: Verses 7.65; 7.73 7.85; 11:50, 11:61, 11:84, 26.36; 26.106; 26.124; 26.142; 26.161; 27.45; 29.36; 46.21.
Obviously these prophets were not real “brothers.” Prophets wives are also called “mothers” of the faithful (33.6). Mary is called “daughter of Imran” because Imran was her family name. As Mary was also a descendant of Aaron, the term “Sister of Aaron” is used as an expression of surprise. From the earliest Islamic history, no Muslim ever thought that Mary was literally the sister of Aaron.
There is a Hadith that one day the prophet entered his wife Safiya’s room and saw her crying,when he asked her the reason she answered: ” I have been told that Aisha and Hafsa are speaking badly of me. They say that they are better than Safiyya because they are cousins and wives of Allah’s Messenger”. The prophet then said: ” how about saying to them”: ” How come when my father is Aaron, my uncle is Moses and my husband is Muhammad?”
It is obvious that these were just expressions, and nobody took them literally.