I am called a son (singular) of my parents and I have brother and a sister.
One of England’s finest scholars, Richard Bauckham, offered a comprehensive analysis of this text in the Catholic Biblical Quarterly. When “son of Mary” is used in Mark 6:3, Bauckham contends that calling Jesus by his mother’s name (a metronymic) indicates that locally, there was knowledge that Jesus was to be distinguished from his “brothers and sisters” because the locals knew that these adelphoi had different mothers. Bauckham cites various Old Testament genealogies where sons are ‘sons of a woman’ (named after their mother, not their father) to support this view.
NIV Zechariah 12:10 "And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son.
(A sword shall pierce your own soul Luke ch1)
orthodoxanswers.org/eureka/mp3/didmaryhave.pdf
When Jesus was found in the Temple at age twelve, the context suggests that he was the only son of Mary and Joseph. There is no hint in this episode of any other children in the family (Luke 2:41–51). Jesus grew up in Nazareth, and the people of Nazareth referred to him as “the son of Mary” (Mark 6:3), not as “a son of Mary.” In fact, others in the Gospels are never referred to as Mary’s sons, not even when they are called Jesus’ “brethren.” If they were in fact her sons, this would be strange usage.
Also, the attitude taken by the “brethren of the Lord” implies they are his elders. In ancient and, particularly, in Eastern societies (remember, Palestine is in Asia), older sons gave advice to younger, but younger seldom gave advice to older—it was considered disrespectful to do so. But we find Jesus’ “brethren” saying to him that Galilee was no place for him and that he should go to Judea so he could make a name for himself (John 7:3–4).
Another time, they sought to restrain him for his own benefit: “And when his family heard it, they went out to seize him, for people were saying, ‘He is beside himself’” (Mark 3:21). This kind of behavior could make sense for ancient Jews only if the “brethren” were older than Jesus, but that alone eliminates them as his biological brothers, since Jesus was Mary’s “first-born” son (Luke 2:7).
Consider what happened at the foot of the cross. When he was dying, Jesus entrusted his mother to the apostle John (John 19:26–27). The Gospels mention four of his “brethren”: James, Joseph, Simon, and Jude. It is hard to imagine why Jesus would have disregarded family ties and made this provision for his mother if these four were also her sons.
catholic.com/library/Brethren_of_the_Lord.asp
Rather, adelphos (Mark 6:3) was used to describe brothers not born of the same parents, like a half-brother or stepbrother.
Nevertheless, other Gospel passages clarify these relationships. James and Joses were the sons of Mary of Clophas (Mk 15:40). Judas was the son of James (not either of the Apostles) (Lk 6:16). James the Lesser was the son of Alphaeus (Lk 6:15). James the Greater and John were the sons of Zebedee with a mother other than our Blessed Mother Mary (Mt 20:20).
According to Jewish law, the oldest son had the responsibility of caring for the widowed mother, and that responsibility would pass to the next oldest if anything happened to the first-born son. By this time, St. Joseph has died. Since Jesus, the first born, had no “blood brother,” He entrusted Mary to the care of St. John, the Beloved Disciple.
Interestingly, the Orthodox Churches solve this problem over brothers and sisters by speculating that St. Joseph was a widower who had other children before he married Mary. These brothers and sisters would really then be half-brothers and half-sisters. Perhaps this notion is why St. Joseph sometimes appears elderly in paintings.
Actually, this whole confusion is not new. About 380, Helvidius suggested that the “brethren” were the children born of Mary and Joseph after Jesus. St. Jerome declared this as a “novel, wicked, and daring affront to the faith of the whole world.” In his On th e Perpetual Virginity of the Blessed Mary, St. Jerome used both Scripture and the fathers, like Saints Ignatius, Polycarp, Irenaeus and Justin Martyr to refute Helvidius. Later, the First Lateran Council (649) definitively declared that Mary was “ever virgin and immaculate.”
Therefore, as Catholics, based on Sacred Scripture and Tradition, we do not believe that Mary and Joseph had other children and consequently that Jesus had blood brothers and sisters.
Fr Saudners
catholiceducation.org/articles/religion/re0090.html