Here are some texts about Jesus' brothers (and sisters!) and the NET translator's notes...
The issue of whether Jesus had
brothers (siblings) has had a long history in the church. Epiphanius, in the 4th century, argued that Mary was a perpetual virgin and had no offspring other than Jesus. Others argued that these brothers were really cousins.
Nothing in the text suggests any of this. (my emphasis)
Jesus’ brothers (really his half brothers) were mentioned previously by John in
2:12 (see the note on
brothers there). They are also mentioned elsewhere in
Matt 13:55 and
Mark 6:3.
With respect to Jesus’
brothers, the so-called Helvidian view is to be preferred (named after Helvidius, a 4th-century theologian). This view holds that the most natural way to understand the phrase is as
a reference to children of Joseph and Mary after the birth of Jesus. Other views are that of Epiphanius (they were children of Joseph by a former marriage) or Jerome (they were cousins).
The tradition of Mary’s perpetual virginity appeared in the 2nd century and is difficult to explain (as J. H. Bernard, St. John [ICC], 1:85, points out) if some of her other children were prominent members of the early church (e.g., James of Jerusalem). But this is outweighed by the natural sense of the words.
Isn’t this the carpenter’s son? Isn’t his mother named Mary?
And aren’t his brothers James, Joseph, Simon, and Judas? Matthew 13:55
Isn’t this the carpenter, the son of Mary and
brother of James, Joses, Judas, and Simon? And aren’t his sisters(!) here with us?” And so they took offense at him. Mark 6:3
All the emphases are mine, of course.