Written by Jimmy Akin, apologist:
here are about ten instances in the New Testament where "brothers" and "sisters" of the Lord are mentioned (Matt. 12:46; Matt. 13:55; Mark 3:31–34; Mark 6:3; Luke 8:19–20; John 2:12, 7:3, 5, 10; Acts 1:14; 1 Cor. 9:5).
When trying to understand these verses, note that the term "brother" (Greek: adelphos) has a wide meaning in the Bible. It is not restricted to the literal meaning of a full brother or half-brother. The same goes for "sister" (adelphe) and the plural form "brothers" (adelphoi). The Old Testament shows that "brother" had a wide semantic range of meaning and could refer to any male relative from whom you are not descended (male relatives from whom you are descended are known as "fathers") and who are not descended from you (your male descendants, regardless of the number of generations removed, are your "sons"), as well as kinsmen such as cousins, those who are members of the family by marriage or by law rather than by blood, and even friends or mere political allies (2 Sam. 1:26; Amos 1:9).
Lot, for example, is called Abraham’s "brother" (Gen. 14:14), even though, being the son of Haran, Abraham’s brother (Gen. 11:26–28), he was actually Abraham’s nephew. Similarly, Jacob is called the "brother" of his uncle Laban (Gen. 29:15). Kish and Eleazar were the sons of Mahli. Kish had sons of his own, but Eleazar had no sons, only daughters, who married their "brethren," the sons of Kish. These "brethren" were really their cousins (1 Chr. 23:21–22).
The terms "brothers," "brother," and "sister" did not refer only to close relatives. Sometimes they meant kinsmen (Deut. 23:7; Neh. 5:7; Jer. 34:9), as in the reference to the forty-two "brethren" of King Azariah (2 Kgs. 10:13–14).
More from him (I didn't want a super long post):
It is unlikely that the term "brother" is being used figuratively or mystically because all Christians are Christ's brothers in that sense, making it pointless to single out certain individuals for this description. Full brother is impossible, as Protestants also acknowledge, since Jesus was not the biological child of Joseph. Half-brother is ruled out by the fact that Mary remained a virgin. It is possible they were adoptive brothers, but there does not seem to be any evidence for this in the biblical or patristic record.
More plausibly, they were step-brothers: children of Joseph who were Jesus' brothers by marriage. There is some evidence for this in the writings of early Christians. The earliest discussion of the matter that we have--in a document known as the Protoevangelium of James (c. A.D. 120)--states that Joseph was a widower who already had a family and thus was willing to become the guardian of a consecrated virgin. Though not inspired, the document was written within living memory of Mary, when Christ's family was still well known, as other sources attest (e.g., second century historian Hegisippus). It may contain accurate traditions regarding the family structure.
The step-brother hypothesis was the most common until St. Jerome (the turn of the fifth century), who popularized the idea that the brethren were cousins. One would not guess this from a casual reading of the New Testament, but many have tried to deduce it from statements in the New Testament.
Part of the issue turns on the meaning of the word "brother." Thus far we have been discussing the English word brother for simplicity. The Greek equivalent (adelphos) includes the same concepts in its range of meaning. But Greek also has a word for "cousin" (anepsios), which seems to have been the normal word used when referring to cousins. An advocate of the cousin hypothesis would need to explain why it wasn't used if Christ’s brethren were cousins.
The standard explanation is that the New Testament isn't ordinary Greek. Some have suggested that parts of it may be translations from Aramaic. It is unknown if or how much of the New Testament had an Aramaic original, but even if none did, Aramaic had a strong influence on it. Probably all the New Testament authors except Luke were native Aramaic-speakers, and much of the dialogue in the Gospels originally occurred in Aramaic. Sometimes the Gospels even tell us the original words (e.g., “Talitha cumi” in Mark 5:41).
This is important because the meaning of the Aramaic word for "brother" (aha) not only includes the meanings already mentioned but also includes other close relations, including cousins.
In fact, there was no word for "cousin" in Aramaic. If one wanted to refer to the cousin relationship, one has to use a circumlocution such as “the son of his uncle” (brona d-`ammeh). This often is too much trouble, so broader kinship terms are used that don’t mean “cousin” in particular; e.g., ahyana ("kinsman"), qariwa ("close relation"), or nasha ("relative"). One such term is aha, which literally means “brother” but is also frequently used in the sense of “relative, kinsman.”
The first Christians in Palestine, not having a word for cousin, would normally have referred to whatever cousins Jesus had with such a general term and, in translating their writing or speech into Greek, it is quite likely that the Aramaic word aha would have been rendered literally with the Greek word for brother (adelphos).
Greek is a very precise language. That is one of the reasons the Holy Spirit saw to it that the scriptures were preserved by it.
Let's take your first example: [Matthew 12:46]ετι δε αυτου λαλουντος τοις οχλοις ιδου η μητηρ και οι αδελφοι αυτου ειστηκεισαν εξω ζητουντες αυτω λαλησαi While he yet talked to the people, behold, his mother and his brethren stood without, desiring to speak with him.
The word highlighted in red means brothers. It is Strong's dictionary #80. adephos (ad-el-fos') here.(Scroll down to #80) It would be very difficult.... reading this verse and knowing what the dictionary says.....to not conclude that these are flesh and blood brethren.
Let's take your next example. [Matthew 13:55] ουχ ουτος εστιν ο του τεκτονος υιος ουχι η μητηρ αυτου λεγεται μαριαμ και οι αδελφοι αυτου ιακωβος και ιωσης και σιμων και ιουδας. Same word....same meaning.
[Mark 3:31–34] Same word again....trust me.
[Mark 6:3] Likewise. [Luke 8:19–20] Again.
The others all show the translation to mean flesh and blood siblings. If the usage were to mean a relative....a cousin, the word would be Anepsios as we find in [Colossians 4:10] Aristarchus my fellow-captive salutes you, and Mark, Barnabas's cousin, concerning whom ye have received orders, (if he come to you, receive him.)Strong's #431. anepsios (an-eps'-ee-os.
As for the rest of your post.....you should not attempt to compare Hebrew to Greek.
ἀδελφός
adelphos
ad-el-fos'
From G1 (as a connective particle) and δελφύς delphus (the womb); a brother (literally or figuratively) near or remote (much like [H1]): - brother
Cousin
συγγενής
suggenēs
soong-ghen-ace'
From G4862 and G1085; a relative (by blood); by extension a fellow countryman: - cousin, kin (-sfolk, -sman).
Seems Brother means, Brother...And,
Cousins means, cousin...