1 cor 9:5 does not If, as you suggest, people are lumped together as "brothers", it would seem strange to exclude Peter (Cephas) from the "brothers of the Lord". -- 9:5 corroborates my point “Brother of the Lord” is clearly a title held by a small number—likely relatives of Jesus—but little more can be said from that. The Gospels of Mark and Matthew name them. Mark 6:3 says that Jesus’ brothers are “James and Joses and Judas and Simon.” Matthew 13:55 has “James and Joseph and Simon and Judas.” At this point the “brothers” appear to be associated with Jesus’ mother Mary and his “sisters.”
It is common even to today in the Middle East and South Asia to refer to one's cousins as ones brothers and sister.
Matthew 12:46 is clearly talking about the universality of the Church in the spiritual sense - as we see a sentence later in whosoever shall do the will of my Father, that is in heaven, he is my brother, and sister, and mother.
BTW-I have checked all the Catholic arguments for this. The argument is normally, "Well, they can be used interchangably and adelphoi is a term of endearment." Of course this argument is toast when you get to some of the ways adelphoi is used in the scriptures (such as his brothers did not even believe Him). Not much endearment there. And then there doesn't seem much endearment of Paul sending greeting to Mark by using the term cousin (anepsio).
Jesus is trying to upset the "Mother of GOD"??