It is Catholic doctrine that Mary was a virgin before, during, and after the birth (ante partum, in partu, post partum) of our Savior; therefore, Jesus is the only child of Mary conceived by the power of the Holy Spirit. The feast in the Catholic liturgical calendar through which we celebrate this miraculous event is the Annunciation; the Archangel Gabriel announced to Mary that she would be the Mother of God and the Holy Spirit would overshadow her, denoting miraculous conception. The Feast is celebrated on March 25. When you add nine months you reach December 25, which of course is the Feast of Christmas, or Christ’s birth.
Often, non-Catholics ask about the phrase “her firstborn son,” since it implies that there were more children afterwards. Today we use the phrase “only son” if there are no other offspring but use “firstborn” to indicate chronological order of more than one child. Ancient Hebrew, however, used the phrase “firstborn” (bekor in Hebrew; prototokos in Greek) to indicate primogeniture (i.e., the custom that the firstborn male would inherit the birthright and all property and authority from the father, regardless of how many children were born). An only child would still have the legal title of “firstborn,” even if there were no second or third child.
Many theologians throughout the centuries wrote beautiful dissertations on the perpetual virginity of Mary. They argued that if you had divinity, Christ, within you, then you would desire nothing else. So as a woman and mother, Mary was totally fulfilled in being the Mother of God. She had attained the highest vocation she could want or ask for. She was a tabernacle of the Lord; our Blessed Savior, the Second Person of the Blessed Trinity, resided in her womb for nine months.
Scripture scholars have also delved into the question of the brothers and sisters of Jesus. It all centers in on the Greek word adelphoi. This word can be translated to mean brothers, cousins, or relatives, such as nephews and uncles. Therefore when we read in Matthew’s Gospel chapter 13:55 concerning the brothers of Jesus, it is ambiguous whether the word adelphos is referring to brothers, cousins, nephews, or uncles. (See Question 31 for more on this point.)
At the foot of the cross, Jesus gave His beloved Mother to the custody of His disciple, John. If Jesus had had a blood brother or sister, He certainly would have given His Mother over to his or her care. Yet, the Scriptures are clear it was to His beloved disciple that Mary was entrusted. With the doctrine of the perpetual virginity of Mary, the lack of scripture to back up the idea that Jesus had siblings, and no part of Church tradition making any references to these relationships, we can conclude Mary had only one child.