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.
HMMMmmm...
Accidently or on purpose??
The man doesn’t supply ALL the instructions; as his design gets modified by the mother’s input as well.