The word for "brothers" in Mark 6:3 is "adelphoi." Adelphoi also appears in Acts 1:15 where it says there were a hundred and twenty "adelphoi." I pity the poor woman who had one hundred and twenty sons! The culture in which Jesus lived used the term "brothers" much more loosely than we do; it could mean cousins, or other close relatives. Check out Galatians 1:19, "I saw none of the other apostles - only James, the Lord’s brother." This verse tells us that there is an apostle named James who is the Lord’s brother. Matthew 10 lists the apostles. There are two James’ - one is the son of Zebedee; the other the son of Alphaeus. Furthermore, Mark 6:3, Matthew 27:56 and John 19:25 taken together show that two of the "brothers" were children of Mary, the wife of Clopas. Mark 6:3 says James and Joseph (footnote n in NIV states: Greek Joses, a variant of Joseph) are "brothers" of the Lord. Matthew 27:56 says Mary, the mother of James and Joses, was at the crucifixion. John 19:25 says that this Mary is the wife of Clopas. Someone might object at this point; what about "until" in Matthew 1:25: "But he had no union with her until (heos) she gave birth to a son." Doesn’t this mean that he did have "union" with her after Jesus was born? The word "until" doesn’t necessitate that the opposite takes place after the "until" is fulfilled. For example: in 2 Samuel 6:23, Michal had no children until (heos) her death. Certainly, she didn’t have any after her death.
I know a bit about Greek, ancient, koine and modern. The NT and Septuagint used the words athelphos and athelphoi to mean both brother/brothers, cousin/cousins and brother or brothers in the sense of "Brother in Christ" Greek then and now is a flexible language. In any event, The Church both at Rome and in the East NEVER believed that the Theotokos had any children after the birth of Christ. The position that she did, and thus was not "Ever Virgin" denies Holy Tradition and is a relatively modern, European born heresy of the Reformation.