Adelphi is used for cousins or close relatives in Deut. 23:7; Neh. 5:7; Jer. 34:9.
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).
Neither Hebrew nor Aramaic have a special word meaning “cousin,” speakers of those languages could use either the word for “brother” or a circumlocution, such as “the son of my uncle.” But circumlocutions are clumsy, so the Jews often used “brother.”
The writers of the New Testament were brought up using the Aramaic equivalent of “brothers” to mean both cousins and sons of the same father—plus other relatives and even non-relatives. When they wrote in Greek, they did the same thing the translators of the Septuagint did.
In the Septuagint the Hebrew word that includes both brothers and cousins was translated as adelphos, which in Greek usually has the narrow meaning that the English “brother” has. Unlike Hebrew or Aramaic, Greek has a separate word for cousin, anepsios, but the translators of the Septuagint used adelphos, even for true cousins.
This same usage was employed by the writers of the New Testament and passed into English translations of the Bible. To determine what “brethren” or “brother” or “sister” means in any one verse, we have to look at the context. When we do that, we see that insuperable problems arise if we assume that Mary had children other than Jesus.
You are confusing Greek for Hebrew. I can find nowhere that the term "cousin" is used in the Hebrew text. Jeremiah 32, refers three times to "Hanamel the son of Shallum your uncle is coming" but it never uses a Hebrew word for cousin.
The writers of the New Testament were brought up using the Aramaic equivalent of “brothers” to mean both cousins and sons of the same father—plus other relatives and even non-relatives.
If this were the case then Paul would never have referred to Mark as Barnabus' cousin.
Greek has a separate word for cousin, anepsios, but the translators of the Septuagint used adelphos, even for true cousins.
Which just underscores why one has to go back to the original Greek and not rely upon the Septuagint. If in John 7 and elsewhere the writer was talking about "cousins", then in the original Greek it would have appear as anepsios. I'm sure John would have known the difference.