There are a number of places in the New Testament (see Mk 3:31-34; 6:3; Mt 12:46; 13:55; Lk 8:19-20; Jn 2:12; 7:3-10; Acts 1:14; and 1 Cor 9:5) where Jesus’ kinsfolk are mentioned using terms such as “brother” (adelphos), “sister” (adelphe) or “brethren” (adelphoi). But “brother” has a wider meaning both in the Scriptures and at the time they were written. It is not restricted to our literal meaning of a full brother or half-brother in the sense of sibling.
Even in the Old Testament “brother” had a wide range of meaning. In the Book of Genesis, for example, Lot is called Abraham’s brother (see 14:14), but his father was Haran — Abraham’s brother (Gn 11:26-28). So, Lot was actually a nephew of Abraham.
The term “brother” could also refer widely to friends or mere political allies (see 2 Sm 1:26; Am 1:9). Thus, in family relationships, “brother” could refer to any male relative from whom you are not descended. We use words like kinsmen and cousins today, but the ancient Jews did not.
In fact, neither Hebrew nor Aramaic had a word meaning “cousin.” They used terms such as “brother,” “sister” or, more rarely, “kin” or “kinsfolk” (syngenis) — sometimes translated as “relative” in English.
James, for example, whom St. Paul called the “brother of the Lord” (Gal 1:19), is identified by Paul as an apostle and is usually understood to be James the Younger. But James the Younger is elsewhere identified as the son of Alphaeus (also called Clopas) and his wife, Mary (see Mt 10:3; Jn 19:25). Even if James the Greater were meant by St. Paul, it is clear that he is from the Zebedee family, and not a son of Mary or a brother of Jesus (in the strict modern sense) at all.
The early Church was aware of the references to Jesus’ brethren, but was not troubled by them, teaching and handing on the doctrine of Mary’s perpetual virginity. This is because the terms referring to Jesus’ brethren were understood in the wider, more ancient sense. Widespread confusion about this began to occur after the 16th century with the rise of Protestantism and the loss of understanding the semantic nuances of ancient family terminology.