Is blood thicker than water? Jesus says here that obedience to his Father in heaven is the strongest bond of all.
Terms that are used commonly over a long period of time tend to lose their meaning and potency. There is, however, a very good reason why those within the church call one another 'brother' and 'sister'. The church, among other things, is a family. And, it is the best family of all. It is a royal family. The royal family of the King of kings and the Lord of lords.
The good news is that this relationship is not through traditional means. It isn't something that we are born into. This family needs to be born again into. We are not brothers and sisters of Jesus and sons and daughters of our Father in heaven because of our blood but because of Jesus' blood.
I have heard people argue that this incident and other incidents that we will look at later show that Jesus is rejecting the notion of family. I disagree. Jesus isn't rejecting family. He is expanding family. Truly, if we regarded one another as brother and sister not just in word but in deed out of obedience to our One Father in heaven, we would be a stronger body of Christ.
And again Jesus tells everyone within the sound of his teaching that knowing the Father's will is not enough. Jesus' family are the ones who DO the will of our Father in heaven. We must be very careful not to fall into the complacent trap of feeling good about our good intentions. In our Father's world, saying what our intentions are doesn't matter, results matter. Those of us who serve in our Father's kingdom are to use our talents to produce fruit. This is the family business.