I returned to the Catholic Church a few years ago.
You can disagree with the following if you like. I'm just sharing this explanation given by the pastor of my church. Assuming I remember the explanation correctly, he said that...
Joseph was a much older widower who already had grown children. At the time, some young girls (Mary being one of them) were selected to remain lifelong virgins. Joseph agreed to marry Mary to care for her - not in the way a man and woman would marry. When Mary was found to be pregnant, at first accusations were made against Joseph.
Because Joseph was so much older, that's why he is not standing at the foot of the cross when Jesus was crucified. (He would've died by that time.)
So, anyway, Joseph's children would be the siblings that everyone keeps referring to.
That's my understanding of it.
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.