Although I have often read the story of Jesus’ identifying his mother and brothers as “Whoever does the will of God” (Matt. 12:46-50; Mark 3:31-35; Luke 8:19-21), I have never noticed until now that Mark offers a unique preface to the story in his gospel.
After Jesus had named his disciples, we read in Mark 3:20-21:
Then he went home, and the crowd gathered again, so that they could not even eat. And when his family heard it, they went out to seize him, for they were saying, ‘He is out of his mind.'”
Immediately after this passage, Mark records the story about Jesus condemning those who blaspheme the Holy Spirit, and then we come back to the story of Jesus identifying his mother and brothers.
Read in this light, it seems that Mark is trying to explain what Jesus does by highlighting the as-yet unbelief of Jesus’ family as opposed to those who do “the will of God.”
No matter your own familial or social connections, Jesus proclaims that they will do you no good unless you repent from your sins and believe the gospel. Not even Jesus’ own family was excepted from this rule.