I must admit I simply can't see any similarity between these phrases. If James had a title "Brother of The Lord" and it meant "Brother in religious affiliation" then Peter and the others were also entitled to it, but are not addressed by Paul in that way.
There are numerous very obvious things wrong with your assumption even before anyone considers what Paul may or may not have regarded as correct grammar and what he may or may not have always rigorously observed as correct spoken or written use of his language.
Firstly, just because he refers to James as "the Lords brother" (not, by the way, as you have it the "brother of the lord") at the end of one particular sentence, that does not mean he is somehow forced to say the same about Peter who he named earlier in the sentence. He may have simply decided to say “lords brother” in one case and not in the other. It may easily be the case that Paul/anyone might sometimes inc. that sort of title or remark, and sometimes he might not.
Sometimes Paul refers to people like Peter and James as “apostles”, and sometimes he doesn’t bother with the term “apostle”. He sometimes refers to himself as an “apostle” also; but mostly he does not.
Secondly, it may have been a specific religious title used for certain members of the faith community, e.g. people baptised by some particular figure such as JtB. In which respect Paul may have been distinguishing one particular “James” amongst several of that name known to that local Jerusalem church group. Or Paul may have wanted to make that sort of distinction for his intended readers in Galatia.
Thirdly, it is certainly true that Paul frequently talks of all sots of people as brothers, brethren and sisters etc., where he almost always means “brothers” in the faith, and not family members. So unless otherwise clarified by Paul, we should probably take any such mention to simply mean brothers in belief … because that’s what he almost always means by that term.
But even apart from all of that - as has been pointed out many times before, both here and in various books - those three words appear at the very end of a sentence which would otherwise have been complete without their addition. That is - the words are added as if in the form of an afterthought. E.g. like this -
“But other apostles saw I none … oh and before I forget, I also saw James … who by the way is “the Lords brother”
The sentence would have been perfectly natural & complete had it simply said “But other apostles saw I none.”
Here is the actual passage from Galatians -
Galatians 1:18-20
“Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. But other of the apostles saw I none, save James the Lord's brother. Now the things which I write unto you, behold, before God, I lie not."
IOW - the way those final few words are added looks suspiciously like it might be a latter addition by Christians who by their later copyist times, thought he would have seen James at that time in Jerusalem. In which respect - keep in mind that the document we are talking about here as Galatians-1 is known only from copies made 150 years or so later by Christian copyists who were apparently well known to have been in the habit of simply altering things where they latter came to think the original needed changing.
Also in that same respect of the few final words being in the form of an added afterthought, and with the possibility of it being a later copyists addition - note that Paul never again says any such thing about James or anyone else being an actual brother of the “Lord”.
Ie - afaik that is the one and only time Paul ever appears to say that anyone might be a family brother of the Christ. And although we have what is supposed to be the written epistles of that very same “James”, nowhere in James own epistle does he himself ever claimed to have ever even met Jesus, let alone ever claim to be his actual family brother for 30+ years.
Keep in mind also that wherever Paul’s letters talk about anyone ever “seeing” Jesus, it is always in the form a visionary spiritual belief, and never as any bodily meeting with a real person named Jesus. Nor does he ever say that this “James” had ever actually seen, met or knew a living Jesus … which is a bit strange if Jesus was supposed to have been known to this James as own brother for over 30 years.
Thus for example, in recounting his vision, Paul says that at some unknown earlier date “James” had also seen a spiritual vision of the “Christ” … but if this Christ was already known to James as his actual brother for 30 years, then it seems a bit unnecessary to say that James knew him from a vision or knew he was the Christ because of any vision.
Also in respect of that same Jerusalem meeting - this supposedly took place only 3 years after the vision which changed Paul’s entire life, such that after the vision he does absolutely nothing else except devote all day every day to preaching about Jesus and what Jesus/God has revealed to him as
THE divine truth of all eternity. And yet when he meets someone supposedly the actual brother of Jesus for the past 33 years, Paul never asks James a single thing about Jesus! And James apparently never offers to tell Paul or anyone else a single thing about him knowing any earthly brother Jesus! This is the most import thing in Paul’s entire existence, to Paul the most important thing in the whole of mankind’s history, and yet he never bothers to ask James a single thing about his brother “Jesus”??
In fact, iirc, elsewhere in his letters (Corinthians, iirc … but I quoted it before anyway) Paul actually says the complete contrary. He says that he is not interested at all in what James or any other so-called “Pillars” of the Jerusalem church claim as their faith in “Christ” (their faith is apparently in the Christ, i.e. a messiah, but not necessarily Paul’s figure “Jesus”). He completely dismisses James and the others as authorities who could tell Paul anything about the “Lord Jesus Christ” … he (Paul) says he knows everything from divine revelation by Christ/God himself/themselves(??), and that whatever James and the other “Pillars” of the church may believe is of no interest or concern to him.
IOW - as I said long long ago; it
is possible that Paul really did think James was a family brother of Jesus. But there are many more reasons to think that one single mention only meant a brother in the faith, if indeed it was not actually just a latter scribal addition.
And that’s apart from the fact that Paul would not necessarily know if James or anyone was really a family brother of this “Christ” anyway. James or anyone else might have claimed to have been a brother or friend of Jesus, without it actually being true. Apart from this particular “James”, it would hardly be surprising if by the time of Paul’s writing, whether that was as early as 50AD or as late as
dejudge thinks it was in the 2nd century (and where in the extant copies that we/everyone is relying upon, it is in any case almost certainly c.200AD, and possibly later than that), all sorts of people had probably claimed to have personally known Jesus or claimed “he was my brother!”.
