Introduction

There are some Christian confessions that claim Mary did not remain a virgin, or that she had other children besides Jesus. Critics often point to verses that refer to the "brethren" or "brother(s)" of Jesus. . This is misguided because the word brother is used in four ways by Holy Scripture. It is used to denote a relationship that can either be by nature, race, kindred, or affection. An example of nature, would be the biological brothers of Peter and Andrew. An example of race, would be all Jews referring to each other as brothers (Deut. 15:12). An example of kindred, would be those within a tribe, clan or family such as Abraham referring to Lot(actually a nephew) as brother (Gen. 13:8). An example of affection, would be Paul writing to the Church in Corinth(1 Cor. 5:11). Mary's perpetual virginity is something that has always been held as a truth of the faith since apostolic times.

Catechism
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".
- Catechism of the Catholic Church, 2nd ed., (1997), n.499
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.
- Catechism of the Catholic Church, 2nd ed., (1997), n.500
Jesus is Mary's only 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 formation she co-operates with a mother's love."
- Catechism of the Catholic Church, 2nd ed., (1997), n.501
Scripture

Gen. 29:15 - Laban calls Jacob his "brother" even though Jacob is his nephew. This demonstrates that brother means kinsmen or cousin in this case.

Deut. 23:7; 1 Chron. 15:5-18; Jer. 34:9; Neh. 5:7 - Here the word "brethren" obviously denotes kinsmen. Hebrew and Aramaic have no word for "cousin."

Mt. 1:25 - Joseph knew her "not until" which is heos in Greek. Heos references the past, never the future. So "not until" does not mean he "knew" her after.

Lk. 2:7 - Jesus being referred to as the "firsborn" does not suggest other children. Jesus was called "only begotten" meaning he had no brothers by nature.

Lk. 22:32 - Jesus tells Peter to strengthen his "brethren." In this case, we clearly see Jesus using "brethren" to refer to the other apostles, not his biological brothers.

Jn. 19:27 - Mary had no other children. Jesus entrusted her to John where Jewish custom demands the biological son.

Acts. 1:12-15 - The gathering of Jesus' "brothers" amounts to about 120. Mary did not give birth to 120 sons. Brother means kinsmen in Hebrew.

Rom. 9:3 - Paul uses "brethren" and "kinsmen" interchangeably. "Brothers" of Jesus does not prove Mary had other children.

Tradition & Fathers
"And indeed it was a virgin, about to marry once for all after her delivery, who gave birth to Christ, in order that each title of sanctity might be fulfilled in Christ's parentage, by means of a mother who was both virgin, and wife of one husband. Again, when He is presented as an infant in the temple, who is it who receives Him into his hands? Who is the first to recognize Him in spirit? A man just and circumspect,' and of course no digamist, (which is plain) even (from this consideration), lest (otherwise) Christ should presently be more worthily preached by a woman, an aged widow, and the wife of one man;' who, living devoted to the temple, was (already) giving in her own person a sufficient token what sort of persons ought to be the adherents to the spiritual temple,--that is, the Church. Such eye-witnesses the Lord in infancy found; no different ones had He in adult age."
- Tertullian of Carthage, On Monogamy (A.D. 160-240)
"For if Mary, as those declare who with sound mind extol her, had no other son but Jesus, and yet Jesus says to His mother, Woman, behold thy son,' and not Behold you have this son also,' then He virtually said to her, Lo, this is Jesus, whom thou didst bear.' Is it not the case that every one who is perfect lives himself no longer, but Christ lives in him; and if Christ lives in him, then it is said of him to Mary, Behold thy son Christ.' What a mind, then, must we have to enable us to interpret in a worthy manner this work, though it be committed to the earthly treasure-house of common speech, of writing which any passer-by can read, and which can be heard when read aloud by any one who lends to it his bodily ears?"
- Origen of Alexandria, Commentary on John (A.D. 184-253)
"And when he had taken her, he knew her not, till she had brought forth her first-born Son.' He hath here used the word till,' not that thou shouldest suspect that afterwards he did know her, but to inform thee that before the birth the Virgin was wholly untouched by man. But why then, it may be said, hath he used the word, till'? Because it is usual in Scripture often to do this, and to use this expression without reference to limited times. For so with respect to the ark likewise, it is said, The raven returned not till the earth was dried up.' And yet it did not return even after that time. And when discoursing also of God, the Scripture saith, From age until age Thou art,' not as fixing limits in this case. And again when it is preaching the Gospel beforehand, and saying, In his days shall righteousness flourish, and abundance of peace, till the moon be taken away,' it doth not set a limit to this fair part of creation. So then here likewise, it uses the word "till," to make certain what was before the birth, but as to what follows, it leaves thee to make the inference."