There's been something I've been ruminating over, perhaps some of you can contribute your thoughts so bear with me a moment. As Stubborn stated, the "brethren of the Lord", though Greek "adelphoi", is believed to be a literal translation of whatever the Aramaic/Hebrew is for brother, with its wider semantic application (brother, cousin, etc.).
However, in apologetics I've also used the argument that when the translators of the Septuagint translated "alma" from the Hebrew (which can mean virgin or maiden), they translated it specifically as "parthenos" which means "virgin". Their translation, then, was a key to the original intent of the passage: namely, when they were permitted to use a language that actually *had* the semantic distinction, they made use of it. If they hadn't used it, and just used a word that meant "maiden", it could be argued that they were showing that no literal virginity was implied in the passage.
Aren't the alma and "brothers of the Lord" argument mutually exclusive? That is, if the LXX translators chose the best Greek word to fit the Hebrew *intent*, and not necessary the literal word, why would the Gospel writers not have done the same and used the Greek word for "cousin" instead of "brother". I suppose you can argue it's a Semiticism...and TantumErgo said it is common NT usage in post #12. But the whole thing rather confuses me, because at the face of it, it looks a bit a priori.
It's because of these difficulties that (despite being a staunch Latin) I tend more toward the Eastern tradition that St. Joseph had childred by a previous marriage. But I'm open to change my mind :) Just trying to shore up the apologetic here so as not to get caught with our pants down.
"That is, if the LXX translators chose the best Greek word to fit the Hebrew *intent*, and not necessary the literal word, why would the Gospel writers not have done the same and used the Greek word for "cousin" instead of "brother"."
Good question, but it could be equally asked of the LXX using the word for "brother" to describe Abraham and Lot. Why would they do it unless they intended carrying over the semiticism into the Greek?
Bearing in mind that the authors of the LXX and the NT were all semites, and thought like semites - with a "non-nuclear" view of family relationships - we are probably demanding a bit much of them to have anticipated the objections of English speaking moderns some 2000 years later.
All I can suggest for further re-inforcement of the Tradition is to speak with an Arab and ask him how many brothers he has. I am not being flippant here - I know an Israeli Arab and I also am familiar with a congregation of Melkite Arabs. Once you get them started, it all makes perfect sense! (Getting them to stop can be another thing entirely!)