I agree Joseph didn't bring the brothers and sisters of Christ into the marriage....but the question then begs to be asked....where did they come from if not from Joseph and Mary?
I’ve heard this answered on EWTN. The term “brethren” meant an extended family: cousins, uncles, etc. in Jesus’ earthly time and place. In some countries this is still practiced.
Full Question My mother told me years ago that shortly before St. Joseph was betrothed to Mary, he had been married to a woman named Melcha who bore him a lot of children. Is this true, and where does this story come from?
Answer
The story about St. Joseph being married to a woman named Melcha with whom he had six children is taken from apocryphal writings, which have no authority. From the Catholic Encyclopedia’s entry on St. Joseph.