A new question for me. My answer is that Jesus was the first born son, and, as head of the family, having the right to appoint a protector for his mom, plus the fact that John was in fact there at the cross and Jesus´ brothers were nowhere to be found. As for the Roman Catholic teaching that Jesus had no siblings and Mary remained perpetually a virgin, they claim that Mark 6:3 which mentions his brothers and sisters is actually a reference to his cousins. I am skeptical that cousin is a possible meaning of this word. I never looked at that in detail, as I have always thought the cousin explanation to be patently absurd. Also, it is the same Catholic Church which in the early centuries passed along the tradition that Jude and James were literally the brothers of Jesus. Ockham tells me a later historical revision is not likely the truth.
John Oakes, PhD