I’m certainly no linguistic expert.
However, I find the assertion most illogical.
The translators of a list of translations have their scholarly reasons for saying brothers vs cousins, uncles, etc.
Sibling relationships are rather basic. I know of no language currently which cannot indicate, refer to a sibling when that is intended, needed.
I leave it to better scholars than I to explain more fully on that score.
But the RC perspective is just laughable, to me. Sounds like another exercise in mental gymnastics to contort simple language about basic simple relationships all out of whack.
God was not that poor an author—whatever the language.
>> I know of no language currently which cannot indicate, refer to a sibling when that is intended, needed.<<
And we’ve not talking about a current language are we? So, therefore, if everything is in the scriptures, where does it say “Blood Brothers”?