Here is one of the things that separates the Protestants from the Catholics. "Is this not the carpenter, the son of Mary and brother of James and Joses and Judah and Simon, and are not his sisters here with us?"
This commentary explains the reason Catholics read this passage and still say that Mary was always a virgin. I am not putting this commentary up to start any arguing as I think it highly unlikely that anything I write will change someone's mind on this subject. It is just an FYI sort of thing for anyone who is curious as I was about how they described it.
St. Mark mentions by name a number of brothers of Jesus, and refers in general to his sisters. But the word "brother" does not necessarily mean son of the same parents. It can also indicate other degrees of relationship -- cousins, nephews, etc. Thus in Genesis 13:8 and 14:14 and 16 Lot is called the brother of Abraham (translated as "kinsman" in RSV), whereas we know that he was Abraham's nephew, the son of Abraham's brother, Haran. The same is true of Laban, who is called the brother of Jacob (Genesis 29:15) although he was his mother's brother (Gen 29:15); there are other instances: cf. 1 Chronicles 23:21-22, etc. This confusion is due to the poverty of Hebrew and Aramaic language: in the absence of distinct terms, the same word, brother, is used to designate different degrees of relationship.
From other Gospel passages we know that James and Joses, who are mentioned here, were sons of Mary of Clophas (Jn 19:25). We know less about Judas and Simon: it seems that they are the apostles Simon the Cananaean (Mt 10:4) and Judas the son of James (Lk 6:16), the author of the Catholic Epistle, in which he describes himself as "brother" of James. In any event, it is nowhere said they were "sons of Mary" -- which would have been the natural thing if they had been our Lord's brothers in the strict sense. Jesus always appears as an only son: to the people of Nazareth, he is "the son of Mary" (Mt 13:55). When he was dying Jesus entrusted his mother to St. John (cf. Jn 19:26-27), which shows that Mary had no other children.