That of course is simply speculation...that Joseph was previously married and had other children before he married Mary.
I don’t go there. I don’t speculate. I don’t know. I have no problem believing that Mary and Joseph had a happy normal marriage and that other children followed Jesus’s birth. This, however, conflicts with an ideological view among some that Mary was ALWAYS a virgin and that her marriage with Joseph was never consummated, even though there isn’t a shred of evidence in the Bible remotely suggesting this. The good Lord commands us to be fruitful and multiply and Mary and Joseph did just that.
BTW-—I first learned Jesus had brothers and sisters in bible study class at my church (a Catholic church) and the class was taught by a CATHOLIC priest. The priest simply taught the Bible as it is written and didn’t try to make up things along the way or make any effort to twist words or meanings in order to satisfy any particular dogma or doctrine. I am truly grateful for that.
The information at this link explains more:
http://www.catholic.com/tracts/mary-ever-virgin
Again, we all can agree to disagree. But that’s the explanation that makes the most sense to me.