3 But rather were those disciples brethren; for even those kinsmen would not be brethren were they not disciples: and to no advantage brethren, if they did not recognize their brother as their master. For in a certain place, when He was informed that His mother and His brethren were standing without, at the time He was speaking to His disciples, He said: “Who is my mother? Or who are my brethren? And stretching out His hand over His disciples, He said, These are my brethren;” and, “Whosoever shall do the will of my Father, the same is my mother, and brother, and sister.” Therefore also Mary, because she did the will of the Father.
What the Lord magnified in her was, that she did the will of the Father, not that flesh gave birth to flesh. Give good heed, beloved. Moreover, when the Lord was regarded with admiration by the multitude, while doing signs and wonders, and showing forth what lay concealed under the flesh, certain admiring souls said: “Happy is the womb that bare You: and He said, Yea, rather, happy are they that hear the word of God, and keep it.” That is to say, even my mother, whom you have called happy, is happy in that she keeps the word of God: not because in her the Word was made flesh and dwelt in us; but because she keeps that same word of God by which she was made, and which in her was made flesh.
Let not men rejoice in temporal offspring, but let them exult if in spirit they are joined to God. We have spoken these things on account of that which the evangelist says, that He dwelt in Capernaum a few days, with His mother, and His brethren, and His disciples.
Source: Tractates on the Gospel of John (New Advent)