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