Paul uses the word αδελφη[ν] (adelphee[n] – sister) in 1 Corinthians 9:5 and Romans 16:1 and 16:15. Here are how they’re translated in the NIV
1 συνιστημι δε υμιν φοιβην την αδελφην ημων ουσαν [και] διακονον της εκκλησιας της εν κεγχρεαις
15 ασπασασθε φιλολογον και ιουλιαν νηρεα και την αδελφην αυτου και ολυμπαν και τους συν αυτοις παντας αγιους
1 I commend to you our sister Phoebe, a deacon of the church in Cenchrea
15 Greet Philologus, Julia, Nereus and his sister, and Olympas and all the saints with them
1 Cor 9:5
μη ουκ εχομεν εξουσιαν αδελφην γυναικα περιαγειν ως και οι λοιποι αποστολοι και οι αδελφοι του κυριου και κηφας
Don’t we have the right to take a believing wife along with us, as do the other apostles and the Lord’s brothers and Cephas
Notice the shadiness. “Sister” is translated authentically in Romans, but is translated as “believer” in 1 Corinthians. Αδελφη is always translated as “sister” in the NT except for this one instance. Why is that? It sticks out like a sore thumb. In order to make sense of its use in 1 Cor it should be translated as “sister” or all instances of “brothers” (αδελφοι) should be translated as “believers” in Paul’s letters.
If Paul really did intend for this instance of “sister” to be rendered as “beliver”, then Paul’s phrase “the lord’s brother(s)” should be rendered as “believers of the lord”… αδελφοι του κυριου. This no longer makes James Jesus’ brother in Gal 1:9, but a “believer of the lord”. Paul might be using that word to distinguish between “apostles” who he says are those who had seen the resurrected Jesus (1 Cor 9:1). This would also make sense of Paul’s use of “our sister” in Romans 16:1. She’s not literally Paul’s sister, but a fellow believer.