After this, he went down to Capernaum, he, and his mother, his brothers, and his disciples; and they stayed there a few days.
His mother and his brethren ... This is the first mention of Jesus' brothers in John; and it is clear from  John 7:5 that they did not yet believe in him.  Regarding the question of whether or not these were sons borne by the mother of Jesus, reference is made to my Commentary on Matthew,  Matthew 13:55-56.  It is the conviction of this writer that there is no good reason for understanding "brothers" in this passage in any unusual manner.