Paul calls all followers brothers and sisters.
True, Paul does address fellow believers as brothers but he also uses specific words from time to time.
As I've said before...context is your key to understanding the Word.
Then three years later I went up to Jerusalem to become acquainted with Cephas, and stayed with him for fifteen days. But I did not see any other of the apostles except James, the Lord's brother. Galatians 1:18-19 NASB
The Greek would render it this way if we put it into English, "James, brother of the Lord." In the Greek of the Lord is in the genitive case. Generally, the genitive case indicates one of possession. In this particular instance it is indicating a family relationship due to the structure of the Greek.
James is being identified as the brother of the Lord.
Paul didn’t write any of the Gospels.
Paul’s use of the words *brother* and *sister* in context of the body of Christ is not the same as the context which Jesus’ neighbors used the term.