Didn’t he just show you the scripture that indicated that she knew Joseph and had children?
The Scripture does not SAY Mary knew Joseph and that the “brothers and sisters” were hers. If you don’t buy the “cousins” explanation, there is also the alternative tradition that these were children of Joseph by another woman. But there is also this to consider: a Semitic family was wider than a nuclear family. In American families we are often —even usually-strangers to our First Cousins. But in Middle East, close relatives are real “family.” Witness the mention of Lot as the “brother” of Abraham.
That's "knew" in the Biblical sense Like Adam "knew" Eve and she bore Abel, etc. I know it doesn't mean they were just getting introduced!
See my post #55.