Paul’s Use of "Sibling"

So here are all of the places where Paul uses the word αδελφοι (adelphee – “brothers” or “siblings”) or the singular version of it:

Romans
1:13; 7:1; 7:4; 8:12; 8:29 (Jesus is the firstborn among many brothers); 9:3; 10:1; 11:25; 12:1; 14:10; 14:13; 14:15; 14:12; 15:14; 15:30; 16:1; 16:14; 16:15 (sister); 16:17; 16:23

1 Corinthians (no biological brothers)
1:1; 1:10; 1:11; 1:26; 2:1; 3:1; 4:6; 5:11; 6:5 (believer); 6:6; 6:8;
7:12 (believer); 7:14 (believer); 7:15 (man or woman who is a believer); 7:24; 7:29; 8:11; 8:12; 8:13; 9:5 (believing woman); 10:1; 11:33; 12:1; 14:6; 14:20; 14:26; 14:39; 15:1; 15:6; 15:31; 15:50; 15:5; 16:11; 16:12; 16:15; 16:20

*14:22 actually uses the word “believer” (πιστευουσιν)

2 Corinthians (no biological brothers)
1:1; 1:8; 2:13; 8:1; 8:18; 8:22; 8:23; 9:3; 9:5; 11:9; 12:18; 13:11;

*6:15 actually uses the word “believer” (πιστω)

Galatians
1:2; 1:11; 1:19 (brother of the lord); 3:15; 4:28; 4:31; 5:11; 5:13; 6:1; 6:18;

*6:10 uses the phrase “family of believers” (οικειους της πιστεως)

Phillipians
1:12; 1:14 (brothers in the lord::των αδελφων εν κυριω );
2:25; 3:1; 3:13; 3:17; 4:1; 4:8; 4:21;

1 Thessalonians (no biological brothers)
1:4; 2:1; 2:9; 2:14; 2:17; 3:2; 3:7; 4:1; 4:6; 4:10; 4:13; 5:1; 5:4;
5:12; 5:14; 5:25; 5:26; 5:27

*1:7 actually uses “believers” (πιστευσυσιν)

Philemon
1:1; 1:2 (sister); 1:7; 1:16; 1:20

Deutero-Paul

Colossians
1:1; 1:2; 4:7
(He is a dear brother, a faithful minister and fellow servant in the Lord);
4:9; 4:15;

Ephesians
6:21
(Tychicus, the beloved brother and faithful servant in the Lord::
τυχικος ο αγαπητος αδελφος και πιστος διακονος εν κυριω); 6:23

2 Thessalonians
1:3; 2:1; 2:13(brothers loved by the Lord::αδελφοι ηγαπημενοι υπο κυριου );
2:15; 3:1; 3:6; 3:13; 3:15

1 Timothy
4:6; 5:1; 5:2; 6:2

2 Timothy 4:21

So only in two instances does Paul use the word “sibling” in an unambiguously biological context: Philemon 1:2 and Romans 16:15. Other than that in the vast majority of times (118 times) Paul, Deutero-Paul (Eph, Col, 2 Thes.) and the Pastorals (1 & 2 Tim, [and Titus, but “sibling” isn’t found in that one]), use the word “sibling” to mean a fellow believer.

Above I noted other strange or ambiguous usages of “brother” like in Romans 8:29 where he says that Jesus was the firstborn among brothers. Did Paul mean literal brothers or brothers by faith?

In 1 Cor 6:5; 7:12; 7:14; 7:15 and 9:5 Paul uses “brother” (or “sister”) but the NIV translates it as “believer”. The more obvious interpretation of the NIV translators is at 7:15 where Paul simply says “brother or sister” (ο αδελφος ή η αδελφη:: lit. the brother or the sister) and the translators reveal Paul’s usage of sibling: a believing man or woman.

Later letters by Paul stop using “brothers of the lord” and start using “brothers in the lord” such as Ephesians 6:21, Philipeans 1:14, and possibly Colossians 4:7. But what I think the smoking gun is is at Galatians 6:10 where Paul writes “family of believers”, but actually uses the Greek word for “believers” and not “brothers”. The vast majority of the time Paul is using siblings in the context of the family of believers.

So who knows. Statistically, it’s more likely that Paul meant brother in the context of the family of believers and not an actual blood brother. The two instances of Paul using “brother(s) of the lord” (1 Cor 9:5; Gal 1:19) might be part of the larger context of the family of believers.