Jesus' Brothers or Cousins ?

Anepsios. Anepsios is the koine Greek word for cousin. Paul used it to describe the relationship which existed between Mark and Barnabas [Colossians 4:10], so with its use he was quite familiar.

Yet, Paul specifically does not use anepsios to describe James, the "brother' of Jesus in Galatians 1:19.

The Greek word used most commonly for "brother" is adelphos, and it specifically indicates "sons of the same mother". Cousins, of course, are not sons of the same mother

It can however, when used as an adjective, indicate brotherly, even sisterly, in describing a relationship of character. It is not, however, employed as an adjective, in Matthew 13:55-56a, for just one example, which reads "Isn't this the carpenter's son? Isn't his mother called Mary, and his brothers [adelphoi], James, Joses, Simon, and Judas? Aren't all of his sisters [adelphai] with us?"

Apparently, according to the Bible at least, Jesus had several brothers and sisters [from the same mother by defnition]. Josephus apparently believed the same as he refers to James "the Just" as the brother [son of the same mother] of Jesus.