Brothers of Jesus: a Catholic Response

Jesus did not have biological siblings. 

This is not made up by the Catholic Church. The Church has few points of doctrine or dogma when it comes to interpreting the Scripture on any matter. What does the Church teach about the siblings of Jesus:

• Catechism of the Catholic Church with cited sources at the bottom of that page
• 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.¹⁵⁴ In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it."¹⁵⁵ And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin".¹⁵⁶
  500 Against this doctrine the objection is sometimes raised that the Bible mentions brothers and sisters of Jesus.¹⁵⁷ 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".¹⁵⁸ They are close relations of Jesus, according to an Old Testament expression.¹⁵⁹
  501 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."¹⁶⁰
• ^{154 Cf. DS 291; 294; 427; 442; 503; 571; 1880. (DS is actually the Enchiridion Symbolrum - abbreviated here according to its editors' names)
  155 LG 57. (Lumen Gentium)
  156 Cf. LG 52.
  157 Cf. Mk 3:31-35; 6:3; 1 Cor 9:5; Gal 1:19.
  158 Mt 13:55; 28:1; cf. Mt 27:56.
  159 Cf. Gen 13:8; 14:16; 29:15; etc.
  160 LG 63; cf. Jn 19:26-27; Rom 8:29; Rev 12:17.}

On the Perpetual Virginity of the Blessed Virgin Mary by St. Jerome

What is open to interpretation - possible theories of the reference to "brothers": 

• Joseph's children by a previous marriage - not likely since they would have been mentioned in the Infancy narratives (all that traveling). While this theory has some basis in the protoevangelium, it would be nice if Catholics let it quietly slip away, because the evidence in the inspired Gospels doesn't hold up. 
• The original language used could have meant "other extended family" - akin to "brethren." Maybe. It holds up to scrutiny when you consider the number of times the disciples (the 12 and all the others who followed Jesus) and the apostles (specifically the 12) are referred to as brothers as opposed to the other group. (see references noted above)

Evidence that Jesus did not have brothers:

• No siblings are mentioned during the Infancy Narratives pre-Egypt
• No siblings are mentioned during the return from Egypt. ("Take the child and His mother")
• No siblings are mentioned when Jesus was 12 and stayed behind in the Temple. Were they all born after Jesus was already 12? 
• Mary was given into the care of John - what was wrong with the siblings who had allegedly been with her when He said, "Who are my mother and brothers?" and the variety of other citations used by Protestants to prove that Mary had other children of her own womb. 

Finally, no evidence here, but a thoughtful question: 

Joseph knew that Jesus was the Child of the Holy Spirit - the angel told him that - would he seriously dare to have such physical relations with the woman who carried this precious Child in her womb? Even if, especially if, he didn't fully understand, the lack of understanding alone would put the fear of God into him. 

Just because the Scriptures say, "He did not have relations with her until she gave birth to her firstborn," does not then have to lead to the conclusion that he did have relations with her afterward. 

St. Paul's quotation from Psalm 109:1, "For He must reign 'until' He has put all enemies under His feet.'" (1 Cor.15:25) ---- and then He no longer reigns? Um. Nope. He keeps reigning! 

And firstborn? My firstborn is 9 years old, sitting at the coffee table creating a "Math Wizardry" notebook. He is and always will be my firstborn. He has zero younger brothers and sisters. The lack of siblings does not remove his title as "my firstborn". Nor does a lack of Jesus' siblings strip away His title of Firstborn. 

(This post was written several months back - my son is now a bit older - still my firstborn - and only born ;) )