They are still not identified as children of Mary. They might have been Joseph's from a prior marriage. They might have been cousins whose parents died and were raised as siblings. We don't know. What we do know is that never are their parents identified. Neither James nor Jude identify themselves as a brother of the Lord.
Why was Mary entrusted to John on the Cross? This would have been highly insulting to any siblings and quite probably illegal under Mosaic law.
I have already mentioned that they could be:
1. Mary's own children.
2. Children of Joseph by another marriage. However, my guts tell me that it had to be a marriage AFTER Joseph married Mary. Otherwise, Jesus would not be the firstborn son in the Davidic line and heir to the throne. Primo-geniture was not always practiced in the passing of the kingship, but it was the standard.
3. Friends/associates who are identified by the word of affection, "brother."
4. Cousins. This is not a common usage of the word adelphoi, but I guess it does take place. Can you give me any other additional clear example in the bible for when "adelphoi" is translated "cousin?"
My contention is that, of the various usages, the word "brother" to translate "adelphoi" is the most likely. After all, it is the direct translation and common intent of the word.