Here, Jesus' family arrives on the scene to help Jesus out of a bind.  Jesus has been healing on the Sabbath day and the Scribes and Pharisees have been challenging Him.  There has even been a rumor that He is possessed by unclean spirits.  Enter, His family.

If Jesus is possessed by unclean spirits, then His family would be the ones to take care of Him and allow Him retreat and rest.

Instead, Jesus redefines family for us:

Whoever does the will of God is My brother, sister, mother...

Jesus is saying, not only am I NOT possessed by unclean spirits, but also I am under the Father-ship of One who accepts all into His family.

What does that say about our responsibility to do the will of God.  What rewards do we receive when we listen to God's instruction?