Well differing means one of us doesn’t have the fullness of truth and is probably wrong about a whole bunch of Bible passages then. So it actually is vitally important to address it.
I would be willing to make a sizeable bet on the side of Apostolic Tradition (which even Luther believed in in regard to Mary’s Perpetual Virginity) rather than your ‘knowledge’. Which is in fact based on a fundamental misunderstanding of the word which in English is translated as ‘brother’.
I might ask, for example, why on earth Jesus gave Mary to John to look after for the rest of her life then, if James was Mary’s other son?
Yep!
The three “pillars of the reformation”, Luther, Calvin, and Zwingli, all believed that Mary was the mother of God.
Mother of God
Martin Luther: “In this work whereby she was made the Mother of God, so many and such good things were given her that no one can grasp them… Not only was Mary the mother of Him who is born [in Bethlehem], but of Him who, before the world, was eternally born of the Father, from a Mother in time and at the same time man and God.” (The Works of Luther, English translation by Pelikan, Concordia, St. Louis, Vol. 7, page 572)
John Calvin: “It cannot be denied that God in choosing and destining Mary to be the Mother of His Son, granted her the highest honor…Elizabeth calls Mary Mother of the Lord, because the unity of the person in the two natures of Christ was such that she could have said that the mortal man engendered in the womb of Mary was at the same time the eternal God.” (Calvini Opera, Corpus reformatorum, Braunschweig-Berlin, 1863-1900, Vol. 45, page 348 and 335.)
Ulrich Zwingli: “It was given to her what belongs to no creature, that in the flesh she should bring forth the Son of God.” ( Zwingli Opera, Corpus reformatorum, Berlin, 1905, in Evang. Luc., Op. Comp., Vol.
6, I, page 639.)
The Perpetual Virginity of Mary:
Martin Luther: “ It is an article of faith that Mary is Mother of the Lord and still a virgin… Christ, we believe, came forth from a womb left perfectly intact.” ( Works of Luther, Vol. 11, pages 319-320; Vol. 6, page 510.)
John Calvin: “ There have been certain folk who have wished to suggest from this passage [Matthew 1:25] that the Virgin Mary had other children than the Son of God, and that Joseph had then dwelt with her later; but what folly this is! For the gospel writer did not wish to record what happened afterwards; he simply wished to make clear Joseph’s obedience and to show that Joseph had been well and truly assured that it was God who had sent His angel to Mary. He had therefore never dwelt with her nor had he shared her company… And beside this Our Lord Jesus Christ is called the first-born. This is not because there was a second or a third, but because the gospel writer is paying regard to the precedence. Scripture speaks thus of naming the first-born whether or no there was any question of the second.” (Sermon on Matthew 1:22-25. Published in 1562.)
Ulrich Zwingli: “I firmly believe that Mary, according to the words of the gospel, as a pure Virgin brought forth for us the Son of God and in childbirth and after childbirth forever remained a pure, intact Virgin.” ( Zwingli Opera, Vol. 1, page 424.)