Two things were among the most important to the culture of Jesus's time: honor and family. Jesus was being accused of having an impure spirit (a goofy claim that he immediately debunked), but his family probably perceived that he was causing them shame by being subject to such a claim. To this Jesus made a pretty radical claim: his family was a new one, formed by God. How his earthly family would have taken this comment is unclear, but it probably wouldn't have made them very happy. For those around him, though, it was the early sign of a greater Kingdom: being part of the family of God.