You appear not to be following too closely here. Christ was not a blood relation to Joseph, he was begotten of God. If his brethren were full siblings then either they would have been begotten of God, and would be God themselves, or Christ would have been begotten of Joseph, and not God.
You are the one not following as I have always said Joseph and Mary's children would be half brothers and sisters....not full siblings.
I realize that destroys your false theology. So be it!