Luke 22:32 - Jesus tells Peter to strengthen his "brethren." In this case, we clearly see Jesus using "brethren" to refer to the other apostles, not his biological brothers.
Acts 1:12-15 - the gathering of Jesus' "brothers" amounts to about 120. That is a lot of "brothers." Brother means kinsmen in Hebrew.
Acts 7:26; 11:1; 13:15,38; 15:3,23,32; 28:17,21 - these are some of many other examples where "brethren" does not mean blood relations.
Rom. 9:3 - Paul uses "brethren" and "kinsmen" interchangeably. "Brothers" of Jesus does not prove Mary had other children.
Gen. 11:26-28 - Lot is Abraham's nephew ("anepsios") / Gen. 13:8; 14:14,16 - Lot is still called Abraham's brother (adelphos") . This proves that, although a Greek word for cousin is "anepsios," Scripture also uses "adelphos" to describe a cousin.
Gen. 29:15 - Laban calls Jacob is "brother" even though Jacob is his nephew. Again, this proves that brother means kinsmen or cousin.
Deut. 23:7; 1 Chron. 15:5-18; Jer. 34:9; Neh. 5:7 -"brethren" means kinsmen. Hebrew and Aramaic have no word for "cousin."
2 Sam. 1:26; 1 Kings 9:13, 20:32 - here we see that "brethren" can even be one who is unrelated (no bloodline), such as a friend.
2 Kings 10:13-14 - King Ahaziah's 42 "brethren" were really his kinsmen.
1 Chron. 23:21-22 - Eleazar's daughters married their "brethren" who were really their cousins.
Neh. 4:14; 5:1,5,8,10,14 - these are more examples of "brothers" meaning "cousins" or "kinsmen."
Tobit 5:11 - Tobit asks Azarias to identify himself and his people, but still calls him "brother."
Amos 1: 9 - brotherhood can also mean an ally (where there is no bloodline).
