This differs from the use of the words Adelphos/Adelphoi more universally to refer to cousins, step-brothers, or me saying you and I are Adelphos/Adelphoi in Christ.
Then why does Christ Himself use the term “
adelphos” as a synonym for “neighbor” and 'loved ones" in Matt. 5:43-47? Here, Jesus is using it in the Jewish, tribal sense. That Jewish sense was very broad, referring to any male relative, not just your literal siblings. The NT uses “
adelphos” to translate the Hebrew and Aramaic word “ah,” which referred to any male relative. For example, in Genesis 14:14, Lot is called Abraham’s “brother,” when Lot was really Abraham’s nephew. The same is true in Gen. 29, were Laban calls Jacob his “brother,” when he is really his nephew. If you want solid proof of this Jewish usage, look at Genesis 27:29 and compare it to Gen. 27:37; here, Isaac gives his blessing to Jacob (thinking he is his brother Esau) by proclaiming: “Be master to your brothers (plural), and may your mother’s sons (plural) bow down to you.” Seems pretty straight-forward, eh?

But, now ask yourself this: How many “brothers” did Jacob have? How many other “sons” did Jacob’s mom Rebecca give birth to?? Only one: Esau himself. So, why then is Isaac speaking in the plural?? V. 37 tells us. Isaac is referring to the entire tribe - to all the male relatives of Jacob. “Brother” was not used literally among the Jews. The term was a very broad and far-reaching one; and comparing Matt 13:55 & Mark 6:3 with Matt 27:56, 61, & 28:1 with Mark 15:40, 47, and 16:1 (and John 19:25) proves that this is how the term “brother” is being applied to the male relatives of Jesus. One cannot escape this fact without ignoring the comprehensive witness of Scripture itself.
See above.
Finally the word Desposyni that appears in Mark 3:21 was a word used to refer to the blood relatives of Jesus and was well known through the first century and a half.
You again reveal a profound ignorance of Greek, for the term “
desposyni” does not appear in Mark 3:21 at all. Rather, the term here is an idiom which literally reads “
hoi par autou” – i.e., “those from his side” - term which some modern translators assume should be rendered as “relatives,” but which more likely (given the context of the passage) refers to His surrounding disciples, in contrast to the scribes and Pharisees. Taken in context, and beginning in v. 20, the passage reads:
“He came home (i.e., to Peter’s house). Again, the crowds gathered, making it impossible for THEM even to eat. When those from His side (
hoi par autou) perceived this, they set out to seize Him, for they said, 'He is overwrought (or ‘beside Himself’ - 'exeste; in Greek).” The scribes who had come from Jerusalem said, ‘He is possessed by Beelzebub,’ and ‘By the prince of demons He casts out demons.’
Then, from verses 23 to 30, Jesus replies to the scribes to show that He is not possessed. And, only after this, in verse 31, does it say:
“His mother and His brothers arrived. Standing outside, they sent word TO HIM and called Him. A crowd seated around Him told Him …”
This alone shows that the people described as “
hoi par autou” (“those from His side”) in v. 21 CANNOT BE the same as His blood relatives, i.e., His “brothers”, since they do not even show up on the scene until verse 31 - 10 verses later.
Now, with that said, there are verses of Scripture which use a word for “relative” (“
suggeneis” in Greek), and not the Jewish term “brother” or “sister” (i.e., “
adelphos” or “
adelphe”). Examples of this can be found in Luke 1:36, Luke 2:44, Colossians 4:10, Romans 16:7, etc. But, without exception, the use of the word “
suggeneis” (“relative” / “kinsman”) always applies to someone who is related to you, but who you do not actually live with as part of your extended family.

Elizabeth was the “relative” of Mary (Luke 1:36), but she did not come from the same household, or even the same tribe as Mary (Mary was of the house of David; Elizabeth was a Levite - see Luke 1:5).
Likewise, the “relatives” of Joseph and Mary in the caravan on their pilgrimage to Jerusalem (per Luke 2:44) would have been from all over Judea and Galilee, not from their extended family back in Nazareth. Likewise, Mark and Barnabas (per Col 4:10) were related, but not of the same household, since Mark was a native of Jerusalem and Barnabas was from Cyprus (see Acts 5:36); and the Andronicus and Junia, who are called the “relatives” of Paul in (Romans 16:7) were Jews from Rome, whereas Paul himself was from Tarsus in Cilicia (modern Turkey).
So, the term “brother” was only used to refer to those who were part of Jesus’ literal, extended family - those with whom He grew up and had intimate family relationships with. This is how the term is used all throughout Scripture.
When Mark referes to Jesus Desposyni in 3:21 he later clarifies who they are in 3:31 when they arrive to retrieve HIM from the house where he was preaching.
No. See above. Jesus’ “brothers” do not arrive until after Jesus rebukes the scribes for claming that He is possessed. By this time, there was no need to seize Him, nor do the brothers try to. Rather, they send word to Him, which is very different. I’m sorry, but you are very sloppy with your exegesis, and with your Greek. The term “
desposyni” does not appear in Mark 3:21.
continued. . .