In ancient Jewish culture, the terms “brother: and “sister” were applied not only to children of the same parents but also to other relatives. In Genesis 14:16, 29:15 and Leviticus 10:4, for example, we know from the context that these passages refer to a relative other than a brother, even though the Hebrew term for “brother” is actually used.
In a similar way, soon after the mention of Jesus’ “brothers” and “sisters,” Mark’s gospel refers to Herod’s half-brother Philip as his “brother” (see 6:17). The first Christians also spoke of each other as “brothers,” even when they were biologically unrelated (see Acts 15:13).
Meanwhile, when some of these “brothers of the Lord” are named in other biblical passages, they are identified as sons of a different Mary (see Mt 13:55-56; 27:56). So even though we may not know exactly how they are related to Jesus, we do know that they are not children of Mary’s womb.
Some Christians claim that the words “her firstborn son” (Lk 2:7), as applied to Jesus, imply that there must have been other children as well. But in biblical culture, “firstborn” was simply a legal term referring to the child who first “opens the womb” (Ex 13:2). If a child were termed “firstborn” only when other children followed, how could the law of Moses have required that the “firstborn” be consecrated soon after birth, before other children arrived (see Ex 13:2, 12, 15; Lk 2:21-24)?
Finally, when St, Matthew in his gospel says that Joseph “had no relations with [Mary] until she bore a son” (1:25), he does not necessarily imply that such relations followed afterward. In the same way, when Jesus says at the end of this same gospel, “I am with you always, until the end of the age” (28:20), he by no means implies thereby that after the end of the age, he will no longer be with us. Similar uses of the word “until” appear throughout Scripture.