…maybe I was not clear enough, in Scriptures, is the term “brother/brethren” used exclusively for direct brother/sister of the same parent/s and the two+ words term for “cousin” always used when referring to what we term “cousins” in English? (…which would then follow: which term would be normally used, specially by those whose blood relations is not necessarily known–ie.: the Virgin Mary and Elizabeth)
Maran atha!
Angel
The answer is no, the Greek word translated as brother or brethern is
adelphos and it is not used exclusively to mean a sibling brother or even cousins. For instance in Galatians 1:19 Paul writes that while he was in Jerusalem visiting with Peter:
“But I saw none of the other apostles except James the Lord’s brother.” [Ga 1:19]
The word translated as brother is the Greek
adelphos but there is something else there. The verse states the James that Paul saw was an Apostle. Now there were only two apostles named James. Were either of them the son of Joseph and Mary? Let’s see shall we? In Mt 10:2-3 Matthew tells us about the Apostles. What does he say of the two James? In Mt 10 :2 we learn about one of them:
“The names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zeb’edee, and John his brother;” [Mt 10:2]
Obviously this is not the sibling of Jesus because his father is Zebedee. In fact this is one of the two brothers that Jesus called the “sons of thunder”. So it must be the other Apostle James who is Jesus’ sibling brother, right? Let’s see. Mt 10:3 tells us:
"3 Philip and Bartholomew; Thomas and Matthew the tax collector; James the son of Alphaeus, and Thaddaeus; 4 Simon the Cananaean, and Judas Iscariot, who betrayed him. " [Mt 10:3] OOPS!!! This James is the son of some guy named Alphaeus. Who was Mary married to, Alphaeus or Joseph??? Obviously Paul is using the word adelphos to denote a relationship other than a sibling son of the same parent.
In Matthew’s gospel we see another use of the word
adelphos. In Matthew 23:8 Jesus says to the Apostles:
“But you are not to be called rabbi, for you have one teacher, and you are all brethren.”
There the word brethren is the Greek
adelphos but obviously all the Apostles did not come from the same family so how could they be sibling brothers? They can’t. Again in Mt 28:10 Jesus calls all of the Apostles his brothers:
"Then Jesus said to them, “Do not be afraid; go and tell my brethren to go to Galilee, and there they will see me.” [Mt 28:10]
Again the word brethern is the translation of the Greek
adelphos but were all the Apostles Jesus’ siblings? Of course not. We see the same use of the Greek
adelphos in Luke 22:32 as well as John 20:17 and John 21:23. So obviously
adelphos was not restricted to mean a sibling brother and to do so is terrible exogesis.