In the fourth century a man called Heldvius wrote a book arguing that Mary was not a perpetual virgin and that the brothers and sisters of Jesus recorded in the Gospels were uterine siblings. This ran counter to what Christians had always believed, and St Jerome responded with a book called The Perpetual Virginity of Blessed Mary in which he suggested that they were either cousins on Mary’s side, or children of a previous marriage of St Joseph.
The word used by the Evangelist is adelfos/adelphos. For monoglots who insist that the word brother must mean uterine brother, here is Strong’s defintion:
To state on the basis of no more than one’s own opinion that it has to be uterine brother defies both the dictionary and most of Christian tradition. It also runs counter to usage elsewhere in the Bible and is, were another one needed, an example of the tendency of modern Protestant sects to make it up as they go along (you won’t find Anglicans or Lutherans indulging in this).
For example, in Genesis 13:8 and 14:16, the word adelphos was used to describe the relationship between Abraham and Lot; however, these two men did not share a brother relationship, but one of uncle and nephew. Similarly, Jacob is called the “brother” of his uncle Laban (Gen. 29:15). Kish and Eleazar were the sons of Mahli. Kish had sons of his own, but Eleazar had no sons, only daughters, who married their “brethren,” the sons of Kish. These “brethren” were really their cousins (1 Chr. 23:21–22). So, if we are going to insist on a single meaning for ‘adelfos’ then we are going to find ourselves with a pile of egg on our face.
The excellent Lonely Pilgrim has an excellent post on this and links to his other splendid explorations of the problem. You will note that like all true apologists, he deals with care and in detail with the evidence; not once does he state that it must be as he says because the word in English is ‘brothers’ – as though it has only one meaning in English. When I refer to fellow Christians as brothers in Christ, I am not implying that my late father and mother had lots more children. When my Trades Union colleague refers to his ‘brothers’ in the Union, he is not implying that he is related to them by birth.
When the Holy Family go into exile in Egypt, there are three of them; when they go to present the child in the Temple, there are three of them; when they go to take the young Jesus to the Temple when he is 12, there are three of them. At the crucifixion Jesus does not commend His mother to Her other sons, he commends her to St John, His cousin. Of course, it could be that all those sons were hiding, or that they just happened to have the same names as the sons of Mary and Clopas. Of course, it could also be that the Church and most Christiasn for most of history have it right. The Lutherans are wrong, the Catholics and the Orthodox are wrong, the Anglicans are wrong, but a few monoglot Englishmen with access to a dictionary know better than the people who spoke kione Greek. Or perhaps a humility check is in order for some people?