Matthew 12:46-50
46 While he was still speaking to the crowds, his mother and his brothers appeared outside, wishing to speak with him.
47 (Someone told him, "Your mother and your brothers are standing outside, asking to speak with you.")
48 But he said in reply to the one who told him, "Who is my mother? Who are my brothers?"
49 And stretching out his hand toward his disciples, he said, "Here are my mother and my brothers.
50 For whoever does the will of my heavenly Father is my brother, and sister, and mother." ------------
Christ's family - mother, brothers, etc. - are those who do the will of his heavenly father. We do this, first, and foremost, by believing and confessing that Jesus is the Christ, our Lord, the Son of God.
What else does the bible tell us about following God's will? Christ tells us in Matthew 22:37 that the greatest commandment (ie, statement of God's will) is to love the Lord your God with all your heart, your soul, and your mind. In his first epistle, St. John tells us that we love God by obeying His commands.
Christ tells us specifically that we are to obey the Lord's commands. I submit that is impossible to "believe in" Christ and not believe what he tells us we should do. If we confess Christ is Lord, and we don't think it is necessary to obey His commands ... is he really our Lord?
PS - Many point to the identification of Christ's brothers/bretheren as evidence that Mary did not remain virgin after the birth of Christ. St. Jerome addressed this question in detail in the 4th century. Click the link to read about it if you are interested.
http://www.crossroadsinitiative.com/library_article/749/Perpetual_Virginity_of_Mary_St_Jerome.html
