Do a word search for "brethren" in the New Testament and see often the word is used.
Were all those people blood siblings with each other?
But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit. She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.” All this took place to fulfill what the Lord had spoken by the prophet: “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel” (which means, God with us). When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, but knew her not until she had given birth to a son. And he called his name Jesus.
Psalm 69:8 I have become a stranger to my brothers, an alien to my mother's sons.
Matthew 1:24-25 When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, but knew her not until she had given birth to a son. And he called his name Jesus.
Matthew 12:46-47 While He was still speaking to the multitudes, behold, His mother and brothers were standing outside, seeking to speak to Him. And someone said to Him, “Behold, Your mother and Your brothers are standing outside seeking to speak to You.”
Matthew 13:55 “Is not this the carpenter’s son? Is not His mother called Mary, and His brothers James and Joseph and Simon and Judas?”
Mark 6:2-3 And when the Sabbath had come, He began to teach in the synagogue; and the many listeners were astonished, saying, “Where did this man get these things, and what is this wisdom given to Him, and such miracles as these performed by His hands?”... “Is not this the carpenter, the son of Mary, and brother of James, and Joses, and Judas, and Simon? Are not His sisters here with us?”
John 2:12 After this He went down to Capernaum, He and His mother, and His brothers, and His disciples; and there they stayed a few days.”
John 7:2-5 Now the Jews’ Feast of Booths was at hand. So his brothers said to him, “Leave here and go to Judea, that your disciples also may see the works you are doing. For no one works in secret if he seeks to be known openly. If you do these things, show yourself to the world.” For not even his brothers believed in him.
Acts 1:14 These all with one mind were continually devoting themselves to prayer, along with the women, and Mary the mother of Jesus, and with His brothers
1 Corinthians 9:4-5 Do we not have a right to eat and drink? Do we not have a right to take along a believing wife, even as the rest of the apostles, and the brothers of the Lord, and Cephas?
Galatians 1:19 But I did not see any other of the apostles except James, the Lord’s brother
Strong's Concordance
http://biblehub.com/greek/80.htm
adelphos: a brother
Original Word: ἀδελφός, οῦ, ὁ
Part of Speech: Noun, Masculine
Transliteration: adelphos
Phonetic Spelling: (ad-el-fos')
Short Definition: a brother
Definition: a brother, member of the same religious community, especially a fellow-Christian.
Here is a link to the occurrences of the Greek word *adelphos*.
http://biblehub.com/greek/80.htm
The word *sister* (adelphe) in the Greek is the same.
http://biblehub.com/greek/79.htm
The word used is *brother* not *cousin*.
It can't mean a member of the same religious community in the context in which they occur, because then that would mean every man in Israel could be identified as Jesus' brother. So that would not identify Jesus as anyone in particular's brother.
It's not going to mean *brother in Christ* as that concept was not yet in place and the Jews, who knew Jesus as a Jew and knew His brothers as Jews, would not even begin to understand the new birth and what being in Christ meant.
They didn't even understand who JESUS was, much less being a *brother in Christ*.
The only definition left then, is to mean physical brother.
And it would not be *cousin*.
The word for *relative* that is used for Elizabeth is *suggenes*, not *adelphe*.
http://biblehub.com/greek/4773.htm
Strong's Concordance
suggenes: akin, a relative
Original Word: συγγενής, ές
Part of Speech: Adjective
Transliteration: suggenes
Phonetic Spelling: (soong-ghen-ace')
Short Definition: akin, a relative
Definition: akin to, related; subst: fellow countryman, kinsman.
The angel told oseph to take Mary *as your wife*, not *as your live in housekeeper and don't touch her or else*.
Nobody in those days thought Mary or Joseph were anyone special and had no reason to use the word *brother* in any but the usual way.
*Isn't this the carpenter's son and and aren't his brothers and sisters with us?*
A very reasonable question not requiring the use of *brother* in any other but the natural sense.
The burden of proof is on you to prove that they did not consummate and lived a lie, their marriage after Scripture says they did consummate it and go on to have other children, a blessing from the Lord..
Well, we gotta compare OT Hebrew to NT Koine Greek and the meanings in each language at each time in history before we can make that kind of comparison.