Rev. Rich Lusk pointed out in a sermon that Mark 3 follows the order of exodus: The Jews plot, Jesus crosses the sea, gathers 12 disciples on a mountain, and then enters a house. In context, the house is the tabernacle, where Jesus, the embodied glory of Yahweh, lives.
That means that Jesus’ discourse on the family is a discourse about the members of the house of the Lord. It’s about qualifications for priesthood: Those who do the will of the Father are the brothers of Jesus.