Here is the deal, you simply object to my every post because you like to bust on me. The banality of your points is there for all to see.
When a poster says, "So you believe..." stand by for some unbiblical absurdity designed to derail actual discussion of the topic. The bible is clear, men have been given the role of spiritual leadership with the family and within our local assemblies. Completely off topic.
Another very common usage of G80 refers fellow Jews or fellow countrymen. So if we use "brethren" for fellow Jews or fellow countrymen, then siblings would be used exclusively when fellow believers regardless of sex or ethnicity are in view.
Now when the reference is to fellow Jews who are believers, I think we should stick with brethren, leaving the ambiguity to be resolved by the reader. Thus Acts 9:30 would remain "brethren." (See also Acts 11:1)
There are many, dozens at least, verses where brethren appears in the NASB referring to believers, too many to bother to list.
Here are the “too many to list” references:
Acts 6:3; 15:1, 2,3,23,32, 33, 36, 40; 16:2, 40; 17:10, 14; 18:18, 27; 21:7
Romans 1:13; 7:1, 4: 8:12, 29: 10:1: 11:25: 12:1; 15:14, 30: 16:14, 17;
1 Corinthians 1:10, 11, 26; 2:1; 3:1; 4:6; 6:5, 8; 11:33, 12:1: 14:6, 20, 26, 39; 15:1, 6, 31, 50. 58: 16:11, 12, 15, 20:
2 Corinthians 1:8, 8:1, 23,: 9:3, 5; 11:9, 26; 13:11
Galatians 1:2, 11; 2:4; 3:5; 4:12, 28, 31; 5:11, 13; 6:1, 18;
Ephesians 6:23;
Philippians 1:12, 14; 3:1, 13, 17; 4:1, 8, 21;
Colossians 1:2; 4:15
1 Thessalonians 1:4; 2:1, 9, 14, 17; 3:7; 4:1, 9, 10, 13; 5:1, 4, 12, 14, 25, 26, 27:
2 Thessalonians 1:3, 2:1, 13, 15; 3:1, 6, 13;
1 Timothy 4:6, 6:2;
2 Timothy 4:21
Hebrews and James would require more study to discern whether siblings or brethren is in view.