The use of the term brethren in Paul.
Ecclesiastical and familial relationships.
This is a differentiated list of all the instances of the term brother or brethren in the Pauline epistles. I have also included all instances of sister.
Paul refers to brethren, in the plural and in the sense of any given fellow saints, in the following instances:
1 Corinthians 6.5, 8; 8.12;
In Romans 9.3 Paul broadly refers to his fellow Jews as my brethren, my kinsmen according to the flesh.
He refers to a brother, in the singular and in the sense of any given fellow saint (sometimes male, often either male or female) in the following instances:
Romans 14.10 (×2), 13, 15, 21;
Paul also refers to a sister, in the singular and in the sense of any given fellow saint (always female), in the following instances:
1 Corinthians 7.15; 9.5 (a sister, a wife).
In some of these instances the sense may not be fellow saint so much as fellow human being, or neighbor.
Paul refers to the broad group of saints belonging to the church to which he is writing as brethren or my brethren, in the vocative case, in the following instances:
Romans 1.13; 7.1, 4; 8.12; 10.1; 11.25; 12.1; 15.14, 30; 16.17;
He also occasionally refers to all the brethren, apparently meaning all the saints in a particular locale, in the opening or closing of an epistle:
1 Corinthians 16.20; Galatians 1.2; 1 Thessalonians 5.26.
Pseudo-Paul* refers broadly to the brethren (Ephesians 6.23; 1 Timothy 4.6), to Pudens and Linus and Claudia and all the brethren (2 Timothy 4.21), and to the saints and faithful brethren (Colossians 1.2). Once he gives instructions to treat the younger men as brothers and the younger women as sisters (1 Timothy 5.1-2), and once he admonishes servants not to disrespect their masters just because they are brethren (1 Timothy 6.2).
* The grouping together of these various references under the name of pseudo-Paul is not meant to imply that Ephesians, Colossians, and the pastoral epistles are all to be attributed to one and the same author.
Only twice does he refer to an individual saint as brother in the vocative case, and he does so exactly in the epistle we might expect, that to Philemon:
Philemon [1.]7, 20 (the brother in both cases is Philemon).
Sometimes Paul refers to a narrow group of people as the brethren, in the plural, and it appears to be understood that he means the (group of) brethren of whom I am speaking. That is, Paul has in mind a narrow set of individuals (not as extensive as an entire church), but does not give the names:
Romans 6.14; 1 Corinthians 16.11, 12 (second instance); 2 Corinthians 8.23; 9.3, 5; 11.9; 1 Thessalonians 4.10 (first instance); Philippians 4.21.
Paul also thrice refers to an unnamed brother, in the singular, in the same sense (that is, Paul obviously has a single individual in mind, but he does not give the name):
2 Corinthians 8.18, 22; 12.18.
Paul sometimes refers to each of certain named individuals as a (or the) brother, in the singular:
Romans 16.23 (Quartus the brother);
Paul likewise twice refers to named individuals as the sister, in the singular:
Romans 16.1 (Phoebe our sister);
In Romans 16.15 Paul mentions Nereus and his sister, clearly a sister by blood, not in a metaphorical sense.
In 1 Corinthians 15.6 Paul refers to the more than 500 brethren who saw the risen Lord on one occasion.
Pseudo-Paul* refers narrowly to Tychicus the beloved brother (Ephesians 6.21; Colossians 4.7), Onesimus the faithful and beloved brother (Colossians 4.9), Timothy the faithful brother (Colossians 1.1), and the brethren in Laodicea (Colossians 4.15).
* Again, the grouping together of these various references under the name of pseudo-Paul is not meant to imply that Ephesians, Colossians, and the pastoral epistles are all to be attributed to one and the same author.
In Romans 8.29 Paul calls Jesus the firstborn among many brethren.
In 1 Corinthians 9.5 Paul calls a certain group of ministers the brothers (or brethren) of the Lord.
In Galatians 1.19 Paul refers to James as the brother of the Lord.
Brethren in the sense of fellows of Jesus: Matthew 28.10; John 20.17; Romans 8.29; Hebrews 2.11-12, 17. (In the context of each of these passages, the brethren are related to Jesus, or to Jesus Christ, or to the son, never to the Lord.)