However, verse (21:91) seems to refer to something more, since it indicates that God "made her and her son a sign" and not just the birth. It is striking that elsewhere the Quran again refers to Mary and Jesus as" a sign"
Mary and Jesus were very interrelated and connected to one another. It is very interesting that whilst talking about the prophets in chapter 21 (the prophets), when it comes to Jesus the Quran talks about Mary and then, through her, about Jesus and says:
And (remember) her who guarded her chastity: We breathed into her of Our spirit and we made her and her son a sign for all peoples. {1}
Here the Quran refers to Mary and her son as "a sign for all peoples" Why? A typical interpretation of this verse is given by Allamah Sayyid M H Tabatabai who holds that the sign here is the birth of Jesus from Mary. Therefore, Jesus and Mary are two sides of the same sign. He adds that the fact that Mary is mentioned in the first place alongside the prophets (although she was not a prophet) and then that she and her son are introduced as "a sign", is a great honour for Mary. {2}
However, verse (21:91) seems to refer to something more, since it indicates that God "made her and her son a sign" and not just the birth. It is striking that elsewhere the Quran again refers to Mary and Jesus, refers as" a sign":
And we made the son of Mary and his mother as asign: We gave them both shelter on high ground affording rest and security and furnished with springs. {3}
Footnote:
1) 2:91
2) Al Mizan fi Tafsir al Qura, p. 317.
3) 23:50
Source: Shomali, (Mary, Jesus and Christianity: An Islamic Perspective) p,8.