If Jesus were the Son of God, as Christianity dictates, he would not also have been the son of Joseph; therefore, any issue arising from Joseph and Mary would have been half-siblings.
No, what I've pointed out is that there was no early tradition in the church that Mary was perpetually virgin. That "tradition" came about several hundred years later when certain elements of Gnosticism influenced the church. Some of the main sects of Gnosticism believed that sex was bad and the body was bad. Since Mary was good, by their reasoning, she must have never had sex.The idea that Joseph had children by a previous marriage doesn't work either. The basics are that the kingship of Israel passed to the oldest surviving male child, even if through adoption. If, say, James had been the first child born to Joseph from an earlier marriage, then James would have inherited the "scepter of David," not Jesus.
For centuries many Christians who believed in the perpetual virginity of Mary have argued that the gospel references to brothers are really to his cousins. They base this on the idea that Hebrew uses the same word for brother and cousin but conveniently ignore the fact that one can discern the meaning through context. For example, when you go to a bar and say "I want a drink," the bartender understands it differently from, say, your 10-year old child coming in from the playground and saying "I want a drink" because of the context. The same word used in different contexts can mean different things.
Regardless, the gospels (with the possible exception of Matthew) were written in Greek, which does distinguish the two. The Greek word for cousin, anepsios, is actually used in the New Testament. The authors weren't doing a wooden "look it up the word dictionary" translation into Greek either. The apostle Paul mentions the brother(s) (adelphos)of the Lord in both Galatians and Corinthians, and there is absolutely no doubt that he was writing in Greek, which was probably his primary language. He described Barnabas as the cousin (anepsios) of Mark.
As I pointed out earlier, Josephus calls James the brother of Jesus, even though he could and did distinguish between brother and cousin in other passages.
Finally, as I have pointed out in other threads, Jesus' statments such as "Whoever does the will of my Father in heaven is my brother and sister and mother," are meaningless if brother and sister is replaced by cousin. The Bible is emphatic that Christians are the spiritual brothers and sisters of Christ, not his spiritual cousins.
Even conservative, devout Catholic scholars -- after finding their positions indefensible -- are beginning to accept that Jesus had siblings. That's what the scriptures teach, what early church documents teach, and what early historical documents teach. Again, I can provide linked cites to early writings if you wish -- most are on the internet.
Oops, forgot this one. Sorry. The evidence from scriptures comes from the fact that Jesus was concieved and born while Mary was a virgin. No one else in the history of mankind has been born that way. We therefore deduce that any other children Mary had were half-siblings of Jesus.