An odd observation I made recently was that every time that Paul mentions Cephas, he always either mentions James or mentions the brother(s) of the Lord (or both in Gal 1.19). Any time Paul has a reason to mention Cephas, a reason to mention James always shows up as well. I mean, the only two letters that Paul writes about Cephas are in 1 Corinthians and Galatians, so it is a small sample. But then again, the only time that Paul writes about brother(s) of the Lord are also in those same epistles.
There is one part of Gal 2.6-7 where a “Peter” is mentioned, but the simplest explanation is that this was a scribal gloss/interpolation into the text of Galatians, and/or that Cephas and Peter were not the same person
assuming that Gal 2.7-8 is authentic to Paul. These two names were probably equivocated in later Chrisian writings (like Jn 1.42). While the two names mean roughly the same thing in Aramaic and Greek respectively, logically there's no reason why someone's name would be translated literally between languages. No one started calling Paul “Smalls”, no one started calling James “leg-puller”, and no one started calling John “Grace of YHWH”; and no one called the High Priest Caiaphas “Peter” either.
So it seems to me that, to Paul and his audience, there was some sort of close relationship between Cephas and James, and between Cephas and the “brothers of the Lord” since they are always mentioned in the same context. To me, it doesn't make sense why Paul would mention Jesus' blood brothers any time that he mentions Cephas. Why the connection between Cephas and blood brothers of Jesus? Unless Cephas himself also had some sort of familial relationship with Jesus, under the literal flesh and blood interperetation of “brother(s) of the Lord”.
Let me see if an analogy makes sense. If, instead of Paul writing “shouldn't we have the right to have (or “lead about”) sister-wives (αδελφην γυναικα::adelphen gynaika) like the brothers of the Lord and Cephas?” in 1 Cor 9.5, a modern example might be “…and shouldn't we have the right to have XYZ thing like Congress and Obama?”. We list Obama outside of Congress because he is is in a leadership position much like Congress, but not a part of Congress himself. So a similar logic might be in place for why Paul uses that sentence construction in 1 Cor 9.5. Similarly, if that logic holds, we might be able to say in Gal 1.19 “…and I got acquainted with Obama. I saw none of the other government officials besides John the Congressman”.
On the flip side, if Paul is indeed talking about flesh and blood brothers of Jesus, then the familial view seems like the only valid parsing of Cephas and the brother(s) of the Lord being mentioned in the same context. Cephas would have to be some sort of close family member of Jesus, but not a brother. Maybe an uncle or something? Who knows.
What if the Congress analogy holds? That the “brothers of the Lord” were really like some sort of Christian-specific judging body like Congress or the Sanhedrin? Though I think this would be anachronistic because we don't start seeing signs of a hierarchy in the various churches until towards the end of the 1st century. Hence the obvious church structure implicit in the pseudo-Pauline letters like the Pastorals. However, maybe there is some room for some sort of synthesis to merge the thesis (governing body) and its – in current scholarship – antithesis (literal flesh and blood brothers).
Surely the various churches during Paul's time couldn't have been like a modern day Anonymous. A free flowing anarchy with absolutely no structure. So maybe there wasn't a very detailed and rigid church structure like in the late 1st/early 2nd century and beyond, but logically something had to have been in place. I think that this somewhat structured church might make sense of why Paul always mentions Cephas and brothers of the Lord almost always in one breath. These brothers of the Lord were the temporary leaders until the Lord himself came down riding on the clouds to rouse those who had fallen asleep and take command of the assemblies.
I don't know. Just a thought.