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When trying to understand these verses, note that the term "brother" (Greek: adelphos) has a wide meaning in the Bible. It is not restricted to the literal meaning of a full brother or half-brother. The same goes for "sister" (adelphe) and the plural form "brothers" (adelphoi). The Old Testament shows that "brother" had a wide semantic range of meaning and could refer to any male relative from whom you are not descended (male relatives from whom you are descended are known as "fathers") and who are not descended from you (your male descendants, regardless of the number of generations removed, are your "sons"), as well as kinsmen such as cousins, those who are members of the family by marriage or by law rather than by blood, and even friends or mere political allies (2 Sam. 1:26; Amos 1:9).

Lot, for example, is called Abraham’s "brother" (Gen. 14:14), even though, being the son of Haran, Abraham’s brother (Gen. 11:26–28), he was actually Abraham’s 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).

The terms "brothers," "brother," and "sister" did not refer only to close relatives. Sometimes they meant kinsmen (Deut. 23:7; Neh. 5:7; Jer. 34:9), as in the reference to the forty-two "brethren" of King Azariah (2 Kgs. 10:13–14).

"Brethren of the Lord"

St. Jerome's comments on Mary's virginity

"[Helvidius] produces Tertullian as a witness [to his view] and quotes Victorinus, bishop of Petavium. Of Tertullian, I say no more than that he did not belong to the Church. But as regards Victorinus, I assert what has already been proven from the gospel—that he [Victorinus] spoke of the brethren of the Lord not as being sons of Mary but brethren in the sense I have explained, that is to say, brethren in point of kinship, not by nature. [By discussing such things we] are . . . following the tiny streams of opinion. Might I not array against you the whole series of ancient writers? Ignatius, Polycarp, Irenaeus, Justin Martyr, and many other apostolic and eloquent men, who against [the heretics] Ebion, Theodotus of Byzantium, and Valentinus, held these same views and wrote volumes replete with wisdom. If you had ever read what they wrote, you would be a wiser man" (Against Helvidius: The Perpetual Virginity of Mary 19 [A.D. 383]).

"We believe that God was born of a virgin, because we read it. We do not believe that Mary was married after she brought forth her Son, because we do not read it. . . . You [Helvidius] say that Mary did not remain a virgin. As for myself, I claim that Joseph himself was a virgin, through Mary, so that a virgin Son might be born of a virginal wedlock" (ibid., 21).

Mary: Ever Virgin

The Perpetual Virginity of Blessed Mary by Jerome