Please tell me why the author used two different greek words for brother and sister if, as the Catholics tell us, they weren't brothers or sisters but cousins?
Maybe one denotes males and one females? I'm not a Greek scholar, but if you can clarify the question, I can get you an answer. Remember this, these "Brothers and sisters" of Christ could be step-siblings. These other children are never referred to as being "son of Mary" or "daughter of Mary."
SD