What does Matthew 12:48 mean?

We can combine details from Matthew and Mark's accounts of this story. Doing so sets a clearer scene. Jesus and His disciples are packed into a house with a huge crowd. Because of the large number of people, they have not been able to eat. Jesus' family—Mary and the other sons born to Mary and Joseph—have heard about this (Matthew 12:46). They have come to the house to take charge of Jesus because they believe Jesus is out of His mind (Mark 3:20–21).

Obviously, Mary and Jesus' brothers did not fully understand His role as Messiah, at this point. It's possible, in the case of His half-siblings, they likely don't believe He is the Promised One, at all. Eventually, some of these family members will come to faith in Him as the Christ. Scripture mentions them praying with the disciples after Jesus' death, resurrection, and return to heaven (Acts 1:14).

Jesus responds to the news that His family is outside wanting to speak to Him in a surprising way. He asks, "Who is my mother, and who are my brothers?" He does not reject His attachment to or responsibility for His immediate family. Rather, He uses the moment to radically enlarge the idea of what it means to be a family. He will define family by attachment to His Father in heaven, and not only those related through birth.