The Gospel According to John, chapter 7:2-10 reads:
2 Now the Jews’ Feast of Booths was at hand. 3 So his brothers said to him, “Leave here and go to Judea, that your disciples also may see the works you are doing. 4 For no one works in secret if he seeks to be known openly. If you do these things, show yourself to the world.” 5 For not even his brothers believed in him. 6 Jesus said to them, “My time has not yet come, but your time is always here. 7 The world cannot hate you, but it hates me because I testify about it that its works are evil. 8 You go up to the feast. I am not going up to this feast, for my time has not yet fully come.” 9 After saying this, he remained in Galilee. 10 But after his brothers had gone up to the feast, then he also went up, not publicly but in private.
Three times the text mentions “Jesus’ brothers”, or as the footnote states, it can be translated, “Jesus’ brothers and sisters”. Who are these brothers and sisters?
1. Literally speaking, someone’s brother or sister is a person with the same parents. Since Jesus is uniquely the Son of God (John 3:18), he cannot have any brother or sister in the literal sense. Therefore, these verses cannot be read literally.
2. Someone’s half-brother or half-sister has one parent in common. Is it possible that Joseph and Mary had natural children after Mary gave birth to Jesus? John 19:26-27 reads:
26 When Jesus saw his mother and the disciple whom he loved standing nearby, he said to his mother, “Woman, behold, your son!” 27 Then he said to the disciple, “Behold, your mother!” And from that hour the disciple took her to his own home.
This action of Jesus as he was dying makes no sense if either Joseph were still alive or Mary had other children who would take care of her. So Jesus did not have a half-brother or a half-sister.
3. Someone’s step-brother or step-sister is a child of a parent from a previous marriage. Is it possible that Joseph was widowed and had children before marrying the Virgin Mary? The John 19 passage above shows this would make no sense because if either Joseph were still alive or Mary had other children, they would take care of her. So Jesus did not have a step-brother or a step-sister.
4. In some cultures such as first-century Jewish culture another relative such as a cousin may be called a brother or sister. This is the remaining possibility and must be the meaning of the passage. These brothers and sisters were likely cousins of Jesus.
The conclusion is that Jesus of Nazareth was an only child.