That nun should have been the first person to ask ... but perhaps the question has just occurred to user6941 ...
Please note that this does not belong to "Sacred deposit" of the faith (the depositum fidei).
For the age of Mary when she was married, please see this one possible source: VII. MARRIAGE OF THE BLESSED VIRGIN TO JOSEPH in The Life of the Blessed Virgin Mary, [Bl.] Emmerich, Anne Catherine (1774-1824).
If Joseph was 17 and Mary 14, chances are that this was NOT Joseph's second marriage.
Whence comes the stories that this was a second marriage for Joseph, that Joseph was an old man when he married Mary, that the brothers and sisters of Jesus were the children of Joesph from his other marriage etc?
These come from apocryphal writings and perhaps with good intentions, i.e., in attempt to explain the perpetual virginity of Mary, etc.
Please see: Marriage in St. Joseph | New Advent.
Please see: The Perpetual Virginity of Mary (Jerome) | New Advent. Please note that St. Jerome argues that St.Joseph also remained virgin, and this to me, makes sense.