Many cultures (Serbian, Greek, Arabic, Hebrew etc.) to this date use the term "brothers" and "sisters" for first or even second cousins. In many cultures, children address adults unrelated to the family as "uncle" or "aunt" in place of "Mr" or "Mrs". Thus, for example in Serbian culture a qualification is made only when questioned as to which type of "brothers" they are and the answer is "borther of aunt" (brat od tetke).
It is only in the Western cultural settings that this custom is alien, which is then erroneously interprted leading to grave misunderstanding of the Bible.
Pardon me for waiting so long to reply to your post. I wanted to make sure I had my information straight before I replied. I emailed one of my college professors and his reply will be below your post.
You clearly don't know koine Greek. The fact of the matter is that Greek speaking Christians from the beginning have always held that the "brothers" of Christ you refer to were cousins or, more likely, step brothers. Why would they have not understood their own language? It is just a little irritating to a Greek that native English speaking Christians, most of whom know no Greek (or at best have a "bible college" knowledge of it) and none of whom speak it as their first language, insist on squeezing koine into an English straight jacket.
The most natural meaning of brother in Greek corresponds to that of English. However, the word can be used in a wide range of meaning. For example, Paul often addresses his Jewish audiences as "brothers," as he does at Antioch of Pisidia (Acts 13:15), and the Sanhedrin in Acts 22:1. Here it simply means something like "fellow Jew." The key is to consider the context. In Mark 6:3; 3:31-35; Matt 13:55-56, the words "brothers" and "sisters" occur in the context of his mother and father, making the usual use of the term to indicate immediate family members most likely. More likely, some Christians of the 3rd through 5th centuries were very moved by the Neo-Platonic view that the body is evil, and that spiritual concerns transcend the needs and pleasures of the body, reading back into the text those ascetic values. The spiritually superior deny the body, do not marry, and live a simple lifestyle in a monastery. I think later generations read those values back into the text of the New Testament, so surely Mary the mother of God did not endulge in sexual pleasure, even with her lawful husband. The contexts mentioning the brothers and sisters of Jesus are understood more naturally as brothers than cousins. The latter can be resorted to only when one has bought into the notion of an other worldy spirituality that denies the will of God expressed regarding marriage in Genesis 1-2.