Nothing in the bible indicates they were step brothers...
Also why they never showed Jesus the respect that normally a younger brother would show for an older brother, because they were probably older.
They didn't believe Jesus was the Messiah...They thought he was nuts...THAT is what the bible tells us...
My older brothers were hero`s my younger ones were nuts they, were probably older brothers
IIRC, Joseph (the OT one) didn’t fare too well with HIS brothers, either!