Exodus 13:2,12 - Jesus is sometimes referred to as the "first-born" son of Mary. But "first-born" is a common Jewish expression meaning the first child to open the womb. It has nothing to do the mother having future children.
Exodus 34:20 - under the Mosaic law, the "first-born" son had to be sanctified. "First-born" status does not require a "second" born.
Ezek. 44:2 - Ezekiel prophesies that no man shall pass through the gate by which the Lord entered the world. This is a prophecy of Mary's perpetual virginity. Mary remained a virgin before, during and after the birth of Jesus.
Mark 6:3 - Jesus was always referred to as "the" son of Mary, not "a" son of Mary. Also "brothers" could have theoretically been Joseph's children from a former marriage that was dissolved by death. However, it is most likely, perhaps most certainly, that Joseph was a virgin, just as were Jesus and Mary. As such, they embodied the true Holy Family, fully consecrated to God.
Luke 1:31,34 - the angel tells Mary that you "will" conceive (using the future tense). Mary responds by saying, "How shall this be?" Mary's response demonstrates that she had taken a vow of lifelong virginity by having no intention to have relations with a man. If Mary did not take such a vow of lifelong virginity, her question would make no sense at all (for we can assume she knew how a child is conceived). She was a consecrated Temple virgin as was an acceptable custom of the times.
Luke 2:41-51 - in searching for Jesus and finding Him in the temple, there is never any mention of other siblings.
John 7:3-4; Mark 3:21 - we see that younger "brothers" were advising Jesus. But this would have been extremely disrespectful for devout Jews if these were Jesus' biological brothers.
John 19:26-27 - it would have been unthinkable for Jesus to commit the care of his mother to a friend if he had brothers.
John 19:25 - the following verses prove that James and Joseph are Jesus' cousins and not his brothers: Mary the wife of Clopas is the sister of the Virgin Mary.
Matt. 27:61, 28:1 - Matthew even refers to Mary the wife of Clopas as "the other Mary."
Matt. 27:56; Mark 15:47 - Mary the wife of Clopas is the mother of James and Joseph.
Mark 6:3 - James and Joseph are called the "brothers" of Jesus. So James and Joseph are Jesus' cousins.
Matt. 10:3 - James is also called the son of "Alpheus." This does not disprove that James is the son of Clopas. The name Alpheus may be Aramaic for Clopas, or James took a Greek name like Saul (Paul), or Mary remarried a man named Alpheus.
And regarding the "brethren"
Luke 1:36 - Elizabeth is Mary's kinswoman. Some Bibles translate kinswoman as "cousin," but this is an improper translation because in Hebrew and Aramaic, there is no word for "cousin."
Luke 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.
Acts 1:12-15 - the gathering of Jesus' "brothers" amounts to about 120. That is a lot of "brothers." Brother means kinsmen in Hebrew.
Acts 7:26; 11:1; 13:15,38; 15:3,23,32; 28:17,21 - these are some of many other examples where "brethren" does not mean blood relations.
Rom. 9:3 - Paul uses "brethren" and "kinsmen" interchangeably. "Brothers" of Jesus does not prove Mary had other children.
Gen. 11:26-28 - Lot is Abraham's nephew ("anepsios") / Gen. 13:8; 14:14,16 - Lot is still called Abraham's brother (adelphos") . This proves that, although a Greek word for cousin is "anepsios," Scripture also uses "adelphos" to describe a cousin.
Gen. 29:15 - Laban calls Jacob is "brother" even though Jacob is his nephew. Again, this proves that brother means kinsmen or cousin.
Deut. 23:7; 1 Chron. 15:5-18; Jer. 34:9; Neh. 5:7 -"brethren" means kinsmen. Hebrew and Aramaic have no word for "cousin."
2 Sam. 1:26; 1 Kings 9:13, 20:32 - here we see that "brethren" can even be one who is unrelated (no bloodline), such as a friend.
2 Kings 10:13-14 - King Ahaziah's 42 "brethren" were really his kinsmen.
1 Chron. 23:21-22 - Eleazar's daughters married their "brethren" who were really their cousins.
Neh. 4:14; 5:1,5,8,10,14 - these are more examples of "brothers" meaning "cousins" or "kinsmen."
Tobit 5:11 - Tobit asks Azarias to identify himself and his people, but still calls him "brother."
Amos 1: 9 - brotherhood can also mean an ally (where there is no bloodline).
Mark 6:3 - Jesus was always referred to as "the" son of Mary, not "a" son of Mary. Also "brothers" could have theoretically been Joseph's children from a former marriage that was dissolved by death. However, it is most likely, perhaps most certainly, that Joseph was a virgin, just as were Jesus and Mary. As such, they embodied the true Holy Family, fully consecrated to God.
Luke 1:31,34 - the angel tells Mary that you "will" conceive (using the future tense). Mary responds by saying, "How shall this be?" Mary's response demonstrates that she had taken a vow of lifelong virginity by having no intention to have relations with a man. If Mary did not take such a vow of lifelong virginity, her question would make no sense at all (for we can assume she knew how a child is conceived). She was a consecrated Temple virgin as was an acceptable custom of the times.
Luke 2:41-51 - in searching for Jesus and finding Him in the temple, there is never any mention of other siblings.
John 7:3-4; Mark 3:21 - we see that younger "brothers" were advising Jesus. But this would have been extremely disrespectful for devout Jews if these were Jesus' biological brothers.
John 19:26-27 - it would have been unthinkable for Jesus to commit the care of his mother to a friend if he had brothers.
John 19:25 - the following verses prove that James and Joseph are Jesus' cousins and not his brothers: Mary the wife of Clopas is the sister of the Virgin Mary.
Matt. 27:61, 28:1 - Matthew even refers to Mary the wife of Clopas as "the other Mary."
Matt. 27:56; Mark 15:47 - Mary the wife of Clopas is the mother of James and Joseph.
Mark 6:3 - James and Joseph are called the "brothers" of Jesus. So James and Joseph are Jesus' cousins.
Matt. 10:3 - James is also called the son of "Alpheus." This does not disprove that James is the son of Clopas. The name Alpheus may be Aramaic for Clopas, or James took a Greek name like Saul (Paul), or Mary remarried a man named Alpheus.