Q. If the Blessed Virgin Mary was always a virgin, then who are the brothers and sisters of Jesus in sacred scripture ?
A. There are two possibilities. The oldest view was that St. Joseph had been previously married and was a widower of great sanctity and devotion to God with children. As Mary was dedicated to the temple and virginity, she needed a protector once she reached adolescence. Joseph was chosen to be her husband/brother and so the brothers and sisters of Jesus were actually step siblings.
The second view, put forward by St. Jerome, is that the word in Greek (adelphos) can be translated as brother meaning a literal sibling; or it can be translated to mean a kinsman. St. Paul uses the exact same Greek word in Galatians 1:11 as is used in Mark 6:3 —adelphos.
In the Mark passage the reader could easily and legitimately interpret it to mean literal siblings of Jesus. But, in the Galations passage, it is obvious that Paul is addressing them as kinsman or brethren in the Lord and not as his own literal brothers/siblings. Protestants freely interpret the passage in Mark as sibling and sibling ONLY and will not allow any other interpretation; and yet when it comes to the exact same word in the Galations they interpret it to mean “brothers of the Lord” and not literal siblings. And yet there is no reason why both passages cannot be interpreted to mean something other than literal “siblings.” And this is what the Catholic Church does. At times from the text we know that the word adelphos refers to literal siblings. And at other times adelphos is used to mean kinsman or something more like…”Bro” or “brother of the Lord.” The Protestant rejection of the doctrine that Mary was ever -Virgin (Calvin and Luther accepted this doctrine) causes them to approach the Mark passage with an entrenched bias.
Also, if we start looking into the historical record, we see that the Mark use of adelphos, in this case, must also mean either step-brothers and sisters or kinsman/cousin/relative. It has always been known to Christians that Mary had no other children after Jesus. Even the Reformers all taught and believed in Mary’s Perpetual Virginity. As did the witness of the early writings of the Church Fathers. She was vowed to perpetual virginity (why else would a betrothed woman ask how could she possibly get pregnant) Therefore we understand the Mark 6:3 passage to be referring to kinsmen of Jesus and not siblings of Jesus.