Matthew 13:55-56 and Mark 6:3 indicate that Jesus had natural half-siblings (same mother). In the Old Testament when "brother" refers to someone in the wider clan, such as a cousin, the necessary information about the actual relationship is always given somewhere in the immediate context. In other words, "brother" means brother unless clues in the context point to a more distant kind of relationship. Neither Matthew 13 nor Mark 6 has any clue of that sort. These verses talk about Jesus' (legal) father Joseph, his mother Mary, and his brothers and sisters. That constitutes a nuclear family. Protestants read these verses in the natural sense. In Matthew 1:25 the preposition "until" by itself doesn't imply much, but taken in the context of the overall statement it does. Joseph did not know her (as a husband normally would do) "until" she brought forth a son. If what is expected did not happen "until" some point in time, the presumption is that the expected thing did begin to happen after that point. The verse teaches that Joseph and Mary remained virgins until after Jesus was born, then had a normal marriage. Matthew 13 and Mark 6 add that they had a family. Ever afterwards, the church spoke of Jesus as having "brothers" (e.g. Acts 1:14; 1 Corinthians 9:5). Between the 2nd and the 5th centuries certain sectors of the patristic church imbibed ascetic ideas from the religious environment of North Africa and became prudish about even marital sexuality. It was during those centuries that the legend developed about Mary remaining an ever-virgin.