"Rubbish! You can't come up with a verse, can you?"
It is more important to be identified as Jesus' brothers than Mary's sons. She was a minor player compared to God becoming flesh. The Father could have chosen any woman to be Jesus mother just like He could have caused the stones to cry out. He is the potter all humanity is clay molded by Him to fulfil His purposes.
True, but to establish your point that Mary was not a perpetual virgin, it is *essential* that they be identified as Mary's sons, since "brother" could mean half-brother.
-A8