see post #707.
Written by Jimmy Akin, apologist:
here are about ten instances in the New Testament where "brothers" and "sisters" of the Lord are mentioned (Matt. 12:46; Matt. 13:55; Mark 3:31–34; Mark 6:3; Luke 8:19–20; John 2:12, 7:3, 5, 10; Acts 1:14; 1 Cor. 9:5).
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).