Did you know that Hebrew has no words for distant cousins, so that when the Septuagint was translated into Greek, the words brothers and sisters were used?
Did you know that the writers of the New Testament honored this convention?
The Gospels were not written in Hebrew. They were written in Greek. The word used was translated brother or brethren 346 of 346 times.
The word used for sister is translated sister 24 out of 24 times.
And no, I do not believe the writers of the New Testament used brother and sister rather than cousin in honor of the Septuagint.