By popular demand to keep the mary-go-round running. Here's another explanation of adelphos (brother in greek) from NAB translators.
The translation of the Greek word
adelphos, particularly in the plural form adelphoi, poses an especially delicate problem. While
the term literally means brothers or other male blood relatives, even in profane Greek the plural can designate two persons, one of either sex, who were born of the same parents.
It was adopted by the early Christians to designate, in a figurative sense, the members of the Christian community, who were conscious of a new familial relationship to one another by reason of their adoption as children of God. They are consequently addressed as adelphoi. This has traditionally been rendered into English by brothers or, more archaically, brethren. There has never been any doubt that this designation includes all the members of
the Christian community, both male and female.
USCCB - Preface the Revised Edition of the New Testament
Greek Adelphos (not hebrew ach)
blood brother (same parents, or same one parent)
spiritual brother (born-again, adopted into the family of God)
Folks want more?