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.
Scripture and tradition are both fairly silent as to what happened with Joseph. However, there is no real doubt that he was dead when Christ's ministry began. There is no mention anywhere or any tradition to indicate that Mary might have remarried.
As far as your idea about firstborn status in the Davidic line, there has always been one glaring "peculiarity" and that is that Jesus had no biological relation to Joseph.
The fact remains that there is no scripture or any other early writings that indicate that Mary had other children. Perpetual virginity was never questioned by any of the chief reformers, in fact they emphasized it. For that matter, perpetual virginity does not interfere with Protestantism's main arguments with Catholic teaching.