Chizuk Emunah (Pt 2) Under the Microscope: Chapter 29

Mark 3:31-35, "There came then his brethren and his mother, and standing without, sent unto him, calling him; and the multitude sat about him, and they said unto him, Behold thy mother and thy brethren without seek thee. And he answered them, saying, Who is my mother, or my brethren? And he looked round about on them which sat about him, and said, Behold my mother and my brethren. For whoever shall do the will of God, the same is my brother, and my sister, and mother." The same subject occurs in Matthew, at the close of chapter 12; and in Luke 8:19. It appears from these statements, that his own mother, and brothers, and sister, would not believe in him, and be his disciples, and that he would not go to meet them who were of his own flesh. The allusion to mutual discord between them is confirmed by the statement in John 7:5, "For neither did his brethren believe in him." 

I have already looked at this passage in detail in the section about Matthew. In this passage, Mark is mentioning that the literal mother and brothers of Jesus did not believe in him. Therefore, Jesus called the people who do the will of God the true mother and brothers of Jesus. There is absolutely nothing in the context of the passage to support the idea that the mother and brothers were referring to anyone but Mary and her other biological offspring.

One can argue that these are cousins, even though Greek has different words for brother (άδελφος) and for cousin (ξάδελφος). There is nothing to indicate that they were adopted children, or step-brothers, or any of these other Catholic harmonizations. If we are to take the Bible at its word, we must understand that Mary most certainly did not remain a virgin after the birth of Jesus.