The New Testament writers would have used the words for "cousin" if they had meant "cousin" in passages about Jesus' brothers and sisters
Persistent commentor "Guy Fawkes" / Jim wrote:
"Brothers" does not have to mean uterine brother.
Ken Temple: (with addition comments)
Except when the context demands it.
Also, Luke calls Elizabeth a relative or cousin of Mary - sungenis = συγγενις = kinswoman, relative
The Greek has words for cousin, kinsman/kinswoman and relative.
καὶ ἰδοὺ Ἐλισάβετ ἡ συγγενίς σου καὶ αὐτὴ συνειληφυῖα υἱὸν ἐν γήρει αὐτῆς, καὶ οὗτος μὴν ἕκτος ἐστὶν αὐτῇ τῇ καλουμένῃ στείρᾳ·
"And behold, even your relative Elizabeth has also conceived a son in her old age; and she who was called barren is now in her sixth month" Luke 1:36
sungenis / συγγενις means "kinswoman", "relative"
If the "brothers and sisters of Jesus" were cousins, the NT writers would have used those words.
But it would make no sense for Jesus to be making the spiritual application and saying, "My true cousins are those that do the will of God"
Matthew 12:46-50 and parallels in Luke 8:19-21 and Mark 6:3; see also, John 7:3-10; cf. Matthew 13:55-57
only uterine/blood brothers makes sense.
ανεψιος / anepsios
John Mark is Barnabas' cousin. Colossians 4:10
anepsios = cousin
καὶ Μᾶρκος ὁ ἀνεψιὸς Βαρναβᾶ
Mark, the cousin of Barnabas
That is an even more specific word; so the NT writers would have used those words if the passages of "brothers and sisters" of Jesus meant "cousins".
Your argument is refuted and defeated again.