What exactly did Matthew mean when he is talking about Jesus' brothers in Matthew 12:46? From Matthew 12:49, its clear that he didn't mean Jesus' disciples as his brothers. So who are the Jesus' brothers mentioned there?
In Protestant Christianity, it is understood that Mary was a virgin until the birth of Jesus, but not after.
24 When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, 25 but knew her not until she had given birth to a son. And he called his name Jesus. Matthew 1:24-25 ESV
After the birth of Jesus, then, Joseph and Mary were like any other married couple and would have likely had other children.
Paul refers to someone named "James" whom he refers to as "the Lord's brother".
But I saw none of the other apostles except James the Lord's brother. Galatians 1:19 ESV
Later in Matthew, Jesus arrives in Nazareth, and he is rejected there because that is where he grew up, and people refused to believe he was anything special. They specifically say that we know his father (who was actually only his stepfather), his mother Mary, and four named brothers.
Is not this the carpenter's son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas? And are not all his sisters with us? Where then did this man get all these things? Matthew 13:55-56 ESV
So, Jesus appears to have had four brothers, namely James, Joseph, Simon and Judas. It also indicates that Jesus had sisters, but it does not give a specific number.
I have composed this answer from an answer on this site which was posted for some other question:
When Catholic Church call Mary the 'Blessed Vigin', they mean she remained a virgin throughout her life and there were no brothers to Jesus. The basis for this is:
Firstly because in Aramaic and Hebrew language there is no word for “Cousin” and as such they were either called brothers or with circumlocution. In Septuagint, the Hebrew word that included both brothers and cousins was translated as “adelphos” which in Greek has same narrow meaning like in English “brother” has. Even though the Greek has a separate word for cousin, the translators of Septuagint used the word Adelphos even for true cousins and the same usage was employed by writers of New Testament.
Secondly, when Jesus is found in temple at the age of 12, the context suggest that he was the only son of Mary (Luke 2:41-51). In Mark 6:3 Jesus is referred as “the son of Mary” and not “a son”.
Mark 6:3 Isn’t this the carpenter, the son of Mary and brother of James, Joses, Judas, and Simon?
Also others in Gospels are never referred as sons of Mary, not even when they are called Jesus’ brothers, which is strange in case they were indeed his brothers.
Thirdly, in ancient eastern societies the younger siblings seldom gave advice to older. But we find these Jesus’ brethren giving him advice on couple of occasions like John 7:3-4 and in Mark 3:21-where they sought to restrain him. This also eliminates them being his brethren since, if they were not his brethren then why they were referred as Brethren of Jesus?
Lastly, of all the four brethren named in Bible, consider only James, for similar reasoning can be applied to others also. “James the younger’s” mother is Mary. Bible describes the scene at the foot of Jesus' cross in following verses:
Matt. 27:56 Among them were Mary Magdalene, Mary the mother of James and Joseph, and the mother of the sons of Zebedee.
Mark 15:40 There were also women, watching from a distance. Among them were Mary Magdalene, and Mary the mother of James the younger and of Joses, and Salome.
John 19:25 Now standing beside Jesus’ cross were his mother, his mother’s sister, Mary the wife of Clopas, and Mary Magdalene.
From above we can say that the mother of James and Joseph must be the wife of Clopas. As James is described as son of Alphaeus also at other place in Bible, Clopas and Alphaeus are probably the same person, since Aramaic name for Alphaeus could be rendered in Greek either as Alphaeus or Clopas. So probably James the younger is the son of Mary and Clopas and who is taken as brother of Joseph the Husband of Mary.
Another view on this verse is that, if Jesus had brothers from Mother Mary, then we find something awkward or amiss with this verse. Here all those who were closest to Jesus were there at the foot of the cross except his purportedly-existing brothers. This is something unimaginable by any count of imagination. If Jesus had brothers then they would have been definitely standing here. [I cannot imagine myself to be not there as a brother of Jesus(In case if I were one) especially when my mother and even distantly related women were standing there unafraid of Jews].
John in his gospel gives us another clue that Jesus had no brothers.
John 20:17 Jesus replied, “Do not touch me, for I have not yet ascended to my Father. Go to my brothers and tell them, ‘I am ascending to my Father and your Father, to my God and your God.’” 20:18 Mary Magdalene came and informed the disciples, “I have seen the Lord!” And she told them what Jesus had said to her.
Here as per John’s account in his gospel, Jesus tells Mary Magdalene to go and tell His brothers and she tells this to Jesus’ disciples. If Jesus had brothers Mary would have gone and delivered this message to them and not to disciples.