Isn't there a more straightforward explanation? None of his brothers believed in Him and were not at the crucifixion, John WAS there.
Incorrect, if your brothers (assumption) thought you were nuts (assumption again), and you died a criminal, that still doesn't mean that it's ok to leave the care of your widowed mother to a friend rather than to them
It's not "nice" NOW and then it would have been something a religious Jew like Jesus would never do.
So, Jesus telling John to take care of Mary as his Mother tells you that He had no other brethern with Mary as their mother.