The unbelief of Jesus’s brothers appears only in Mark 3 and John 7, both of which want to portray Jesus as rejected by his own. And Mark 3:21 is a hard phrase that translations render differently.

The translation of Mark 3:21 is debated, and that should be said plainly. But the broader picture of family skepticism is attested in Mark, in John, and in the absence of any family member among the disciples during the ministry, and it runs against the Gospels’ own interests: by the time they were written, James was a revered pillar of the Jerusalem church. Inventing a skeptical past for the church’s most respected leader is not something a propagandist does.