3 “Now the Jews' feast of tabernacles was at hand.” What the feast of tabernacles is, they who read the Scriptures know. They used on the holy day to make tabernacles, in likeness of the tabernacles in which they dwelt while they sojourned in the wilderness, after being led out of Egypt. This was a holy day, a great solemnity. The Jews were celebrating this, as being mindful of the Lord's benefits— they who were about to kill the Lord. On this holy day, then (for there were several holy days; but it was called a holy day with the Jews, though it was not one day, but several), “His brethren” spoke to the Lord Christ.
Understand the phrase, “His brethren,” as you know it must be taken, for it is not a new thing you hear. The blood relations of the Virgin Mary used to be called the Lord's brethren. For it was of the usage of Scripture to call blood relations and all other near kindred by the term brethren, which is foreign to our usage, and not within our manner of speech. For who would call an uncle or a sister's son “brother”? Yet the Scripture calls relatives of this kind “brothers.” For Abraham and Lot are called brothers, while Abraham was Lot's uncle. Laban and Jacob are called brothers, while Laban was Jacob's uncle. When, therefore, you hear of the Lord's brethren, consider them the blood relations of Mary, who did not a second time bear children.
For, as in the sepulchre, where the Lord's body was laid, neither before nor after did any dead lie; so, likewise, Mary's womb, neither before nor after conceived anything mortal.
Source: Tractates on the Gospel of John (New Advent)