Here is Mark:
Mark 3: 13 Jesus went up on a mountainside and called to him those he wanted, and they came to him. 14 He appointed twelve[a] that they might be with him and that he might send them out to preach 15 and to have authority to drive out demons. 16 These are the twelve he appointed: Simon (to whom he gave the name Peter), 17 James son of Zebedee and his brother John (to them he gave the name Boanerges, which means “sons of thunder”), 18 Andrew, Philip, Bartholomew, Matthew, Thomas, James son of Alphaeus, Thaddaeus, Simon the Zealot 19 and Judas Iscariot, who betrayed him….
…Then Jesus entered a house, and again a crowd gathered, so that he and his disciples were not even able to eat. 21 When his family** heard about this, they went to take charge of him, for they said, “He is out of his mind.”
Then Jesus’ mother and brothers arrived. Standing outside, they sent someone in to call him. 32 A crowd was sitting around him, and they told him, “Your mother and brothers are outside looking for you.”
33 “Who are my mother and my brothers?” he asked.
34 Then he looked at those seated in a circle around him and said, “Here are my mother and my brothers! 35 Whoever does God’s will is my brother and sister and mother.”
Mark 6: 2 And when the sabbath day was come, he began to teach in the synagogue: and many hearing him were astonished, saying, From whence hath this man these things? and what wisdom is this which is given unto him, that even such mighty works are wrought by his hands?
3 Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us? And they were offended at him.**
Note that Mark has the crowd saying “the son” instead of “a son”…singular not one of many.
4 But Jesus, said unto them, A prophet is not without honour, but in his own country, and among his own kin, and in his own house.
as you can see, Mark does the same thing as Matt, though Mark emphasizes that the brothers were not believers. So again, the 12 are identified, they are believers and are with Jesus when the unbelieving brothers come to take custody of Jesus. Shortly after having distinguished between the believing 12 and the unbelieving brothers, Mark provides the names of 4 brothers……having already made a very dramatic distinction between the two groups (in chapter 3), Mark would not (in chapter 6) negate that distinction and list members of the believing 12 as belonging to the other group (of brothers) which have been clearly identified as unbelievers.