This is where your man-made belief confuses you. You start with that premise, and you BELIEVE and accept as fact the brothers of Christ mentioned in Scripture were blood related. You want to believe this, because it supports your man-made religion. But there is no Scripture to support your made-up belief. Brother does not mean son of the same mother in Scripture. Christ had at least 500 'brothers' and Abraham and Lot were brothers, according to Scripture.
You are simply wrong, and cannot support your beliefs from Scripture.
Guys, calm down. You’re trying too hard to justify your own position. It was known to the early Church that Joseph was a widower with grown children when Mary was given to him. Not every mention of “brother” in Scripture refers to natural brothers... but it is easily possible that some do.
Therefore, it is entirely possible that Jesus had siblings AND that Mary had no other children. Does this really have to cause to much trouble between us?