2 “He went down,” as the evangelist says, “to Capernaum, He, and His mother, and His brethren, and His disciples; and they continued there not many days.” Behold He has a mother, and brethren, and disciples: whence He has a mother, thence brethren. For our Scripture is wont to call them brethren, not only that are sprung from the same man and woman, or from the same mother, or from the same father, though by different mothers; or, in truth, that are of the same degree as cousins by the father's or mother's side: not these alone is our Scripture wont to call brethren.
The Scripture must be understood as it speaks. It has its own language; one who does not know this language is perplexed and says, Whence had the Lord brethren? For surely Mary did not give birth a second time? Far from it! With her begins the dignity of virgins. She could be a mother, but a woman known of man she could not be. She is spoken of as mulier [which usually signifies a wife], but only in reference to her sex, not as implying loss of virgin purity: and this follows from the language of Scripture itself.
For Eve, too, immediately she was formed from the side of her husband, and as yet not known of her husband, is, as you know, called mulier: “And he made her a woman [mulier].” Then, whence the brethren? The kinsmen of Mary, of whatever degree, are the brethren of the Lord. How do we prove this? From Scripture itself. Lot is called “Abraham's brother;” he was his brother's son. Read, and you will find that Abraham was Lot's uncle on the father's side, and yet they are called brethren.
Why, but because they were kinsmen? Laban the Syrian was Jacob's uncle by the mother's side, for he was the brother of Rebecca, Isaac's wife and Jacob's mother. Read the Scripture, and you will find that uncle and sister's son are called brothers. When you have known this rule, you will find that all the blood relations of Mary are the brethren of Christ.
Source: Tractates on the Gospel of John (New Advent)