Mary living with John and her reply to Elizabeth are found in Luke and are available to all. It is not special to the Catholic Church.
You realize that before the Magnificat prayer got into St. Luke's gospel, St. Luke had to find out from somebody what did the prayer say?
Further, that the Magnificat and the fact that Mary adopted John, and a few others, are in the scripture does not mean that the only thing Sts Luke and John knew about her is wholly contained in the scriptures that they wrote. St. John, in fact, writes that he did not write down everything Chirst did -- what makes you think in the case of Mary all he knew were the Wedding at Cana?