The following is taken from Lumen Verum Apologetics, an on line, 30-part apologetics course by Robert Haddad. www.theworkofgod.org

In the  Bible we read the following passages:

"Where did this man get this wisdom and these deeds of power? Is not this the carpenter's son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas? And are not all his sisters with us? Where then did this man get all this? And they took offense at him" (St. Matt. 13, 54-57).

According to Fundamentalists, therefore, it appears clear that Jesus Christ had brothers and sisters, and that Mary did not remain a virgin all Her life. Yet, the Catholic Church asserts that Mary was a virgin before, during, and remained so perpetually after the birth of Christ (Ante partum, in partu, post partum). Consequently, the Catholic Church teaches a wrong doctrine, it is false, and you should leave Her!

The Catholic answer to this apparent contradiction is detailed but decisive. There existed no special word in Hebrew or Aramaic for "cousin." The word "brother" is used in these languages in a general sense, and does not necessarily imply children of the same parent. There are many examples in the Old Testament when the word brother was applied to any sort of relations: nephew (Gen. 12, 5; 13, 8; 14, 16), uncle (Gen. 29, 15); husband (Songs. 4, 9); a member of the same tribe (2 Kgs. 9, 13); of the same people (Exod. 2, 21); an ally (Amos 1, 9); a friend (2 Kgs. 1, 26); one of the same office (1 Sam. 9, 13).

Another such example is found in Gal. 1, 19, where St. Paul calls St. James "the Lord’s brother." There were only two Apostles named James - James Son of Zebedee, and James the Less Son of Cleophas. Neither had St. Joseph as their father. "Brother" is used here simply to designate the close affection between the Apostle and Our Lord, not to show a blood relationship.

A number of distinguished Catholic commentators, including St. Thomas Aquinas, actually hold that the Blessed Virgin Mary had made a formal vow of perpetual virginity despite being betrothed to St. Joseph. The Jews during the four centuries before Christ had begun to develop a concept of consecrated virginity, particularly in the community of the Essenes. A vow of virginity would help explain why Our Lady was so perplexed after the Angel Gabriel announced to Her that She was about to bear a son. According to Jewish custom at the time, marriage was in two stages. The first stage, or betrothal, was when the marriage was effectively made. Our Lady and St. Joseph had concluded this stage. Sexual relationships after this point were not considered as fornication. Yet we know that nothing of this kind had taken place between Our Lady and St. Joseph: "How can this be, since I am a virgin?" (St. Luke 1, 34). The second stage of marriage was the social formality of the public celebration. Our Lady and St. Joseph in all probability missed out on this second stage due to their flight to Egypt, nevertheless, this fact did not impugn the validity of their marriage.

"But St. Matthew 1, 25 says that Joseph ‘had no marital relations with her until she had borne a son; and he named him Jesus.’ This implies that Mary, therefore, had other children conceived by Joseph after giving birth to Jesus."

It would be well here to reproduce the footnote commentary on this verse from the Douai-Rheims version of the New Testament:

"St. Jerome shows, by diverse examples, that this expression of the Evangelist was a manner of speaking usual among the Hebrews, to denote by the word until, only what is done, without any regard to the future. Thus it is said, Genesis 8, 6 and 7, that Noe sent forth a raven, which went forth, and did not return till the waters were dried up on the earth. That is, did not return anymore. Also Isaias 46, 4, God says: I am till you grow old. Who dare infer that God should then cease to be?...God saith to his divine Son: Sit on my right till I make thy enemies thy footstool. Shall he sit no longer after his enemies are subdued?"

Further, according to the Jewish Law a child was designated as "first-born" irrespective of whether there were yet, or ever to be, subsequent children born to the same mother. This is gathered from Exod. 13, 2, which required that "every first-born that openeth the womb among the children of Israel" be consecrated to God forty days after their birth.

Who, then, exactly were the brothers and sisters of Jesus Christ?

It is best to start by looking at St. John 19, 25. There it is evident that Our Lady had an older sister whose name was also Mary: "Meanwhile, standing near the cross of Jesus were his mother, and his mother's sister, Mary the wife of Clopas, and Mary Magdalene."

Turning next to the Gospel of St. Mark 15, 40, speaking on the same point: "There were also women looking on from a distance; among them were Mary Magdalene, and Mary the mother of James the younger (Less) and of Joses (Joseph), and Salome." Who is this "Mary the mother of James the younger and of Joses?" Of the Marys mentioned in St. John 19, 25 it must be Mary the wife of Clopas, not Mary the "mother of Jesus," as Our Lady is never mentioned by any other title in the New Testament except as "mother of Jesus." Further, we know that the father of James the younger was Clopas, the husband of Mary of Clopas (St. Mark 3, 18), making Mary of Clopas James’ mother. As for Jude, he was also a son of Clopas and Our Lady’s sister as Scripture speaks of him as a brother of James the younger: "James son of Alphaeus (Clopas), and Simon the Zealot, and Judas the brother of James" (Acts 1, 13 [Douai]). Consequently, Our Lord had cousins by the names of James, Joseph and Jude.

One can safely state then that the "brothers" of Our Lord as mentioned in St. Matt. 13, 54-57 being James, Joseph, Jude etc. are in fact the same James, Joseph and Jude just determined to be His cousins. It would be forcing credibility to believe that Our Lady and Her older "sister" both had the same names and also had children with the same names. One can expect, also, that after St. Joseph died Our Lady would have gone with Our Lord to live with or nearby Her older "sister," explaining why She was traveling with those mentioned in St. Matt. 12, 46. It is a clear example of the word "brother" being used to refer to a first or second cousin.

It is also important to examine closely three major events in Our Lord’s life referred to in the Gospels: (i) the return of the Holy Family from Egypt to Nazareth after the death of Herod; (ii) the finding of the Child Jesus in the Temple of Jerusalem after being lost for three days; (iii) Our Lord giving Our Lady to the care of St. John at His crucifixion. Our Lord, according to pious tradition, was 10, 12 and 33 years of age respectively when each of these events occurred. Yet, never is there any mention of brothers or sisters of His being present, which one would naturally expect if they had actually existed.

Finally, the early Protestant leaders - Luther, Calvin, Zwingli, and Beza - likewise accepted that the Blessed Virgin Mary had no other children besides Christ.

The Fathers:

St. Ephrem of Edessa (+373 A.D.)1 :
"...the rod of Aaron that budded, truly have you appeared as a stem whose flower is your true Son, our Christ, my God and my Maker; you did bear according to the flesh God and the Word, did preserve your virginity before His birth, did remain a virgin after His birth, and we have been reconciled to God by Christ your Son."

St. Cyril of Jerusalem (+386 A.D.)2 :
"Of Him Who is God and Man are you the Mother, Virgin before (His) birth, Virgin in birth, and Virgin after birth."

St. Jerome (+420 A.D.), Contra Helvidius, i:
"Suppose that the Brethren of the Lord were Joseph’s sons by another wife. But we understand the Brethren of the Lord to be not the sons of Joseph, but cousins of the Savior, the sons of Mary, his mother’s sister."

St. Augustine of Hippo, Holy Virginity (401 A.D.):
"In being born of a Virgin who chose to remain a Virgin even before she knew who was to be born of her, Christ wanted to approve virginity rather than impose it. And He wanted virginity to be of free choice even in that woman in whom He took upon Himself the form of a slave."

St. Augustine of Hippo (+430 A.D.), De Annunt. Dom. iii:
"It is written (Ezekiel 44, 2): ‘This gate shall be shut, it shall not be opened, and no man shall pass through it. Because the Lord the God of Israel hath entered in by it...’ What means this closed gate in the house of the Lord, except that Mary is to be ever inviolate? What does it mean that ‘no man shall pass through it,’ save that Joseph shall not know her? And what is this - ‘The Lord alone enters in and goeth out by it,’ except that the Holy Ghost shall impregnate her, and that the Lord of Angels shall be born of her? And what means this - ‘It shall be shut for evermore,’ but that Mary is a Virgin before His birth, a Virgin in His birth, and a Virgin after His birth."

St. John Chrysostom (+407 A.D.), Opus Imperf. in Matt., Hom. 1 (?):
"Joseph did not know her, until she gave birth, being unaware of her dignity: but after she had given birth, then did he know her (by way of acquaintance). Because by reason of her child she surpassed the whole world in beauty and dignity: Since she alone in the narrow abode of her womb received him whom the world cannot contain."

Catechism of the Council of Trent (1566):
He is born of His Mother without any diminution of her maternal virginity, just as He afterwards went forth from the sepulcher while it was closed and sealed, and entered the room in which His disciples were assembled, the doors being shut; or, not to depart from every-day examples, just as the rays of the sun penetrate without breaking or injuring in the least the solid substance of glass, so after a like but more exalted manner did Jesus Christ come forth from His mother’s womb without injury to her maternal virginity. This immaculate and perpetual virginity forms, therefore, the just theme of our eulogy. Such was the work of the Holy Ghost, who at the Conception and birth of the Son so favored the Virgin Mother as to impart to her fecundity while preserving inviolate her perpetual virginity.

Catechism of the Catholic Church (1992): Mary - "ever-virgin"

No. 499: The deepening of faith in the virginal motherhood led the Church to confess Mary’s real and perpetual virginity even in the act of giving birth to the Son of God made man. In fact, Christ’s birth "did not diminish His Mother’s virginal integrity but sanctified it." And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin."

No. 500: Against this doctrine the objection is sometimes raised that the Bible mentions brothers and sisters of Jesus. The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, "brothers of Jesus," are the sons of another Mary, a disciple of Christ, whom St. Matthew significantly calls "the other Mary." They are close relations of Jesus, according to an Old Testament expression.

No. 501: Jesus is Mary’s Son, but her spiritual motherhood extends to all men whom indeed he came to save: "The Son whom she brought forth is he whom God placed as the first-born among many brethren, that is, the faithful in whose generation and formulation she co-operates with a mother’s love."