Jesus and His Family
The mother of Jesus and his brothers came to him but were unable to join him because of the crowd. He was told, “Your mother and your brothers are standing outside and they wish to see you.” He said to them in reply, “My mother and my brothers are those who hear the word of God and act on it.”
Luke 8:19-21
When about to enjoin upon others that he who has not left father and mother is not worthy of the Son of God, Jesus who gives Himself as an example to others, first submits Himself to this precept. Not that He denies the claims of filial piety, for it is His own sentence, He that knows not his father and mother shall die the death (Lev 20:9), but because He knows that He is more bound to obey His Heavenly Father's mysteries than the feelings of His mother. Nor however are His parents harshly rejected, but the bonds of the spirit are shown to be more sacred than those of the body. Therefore in this place He does not disown His mother, (as some heretics say, eagerly catching at His speech,) since she is also acknowledged from the cross; but the law of heavenly ordinances is preferred to earthly affection.
They then who hear the word of God and do it, are called the mother of our Lord, because they daily in their actions or words bring Him forth as it were in their inmost hearts; they also are His brethren where they do the will of His Father, Who is in heaven.
References:
Catena Aurea (St. Thomas Aquinas)
Explanations on the New Testament (St. Theophylact of Ochrid)
Commentary on the Gospel according to Saint Luke (St. Ambrose of Milan)
Little-Known Facts about the Holy Land: Mount of Temptation, Sea of Galilee, On the way to Jericho