From what Paul writes in that letter, I think it's highly likely that the half-sentence (which is only 10 words) did not mean a literal family blood brother. The reason why I think that is as follows -
1 that claim (if we are allowed to politely call it a "claim" just for the sake of clarity & explanation here) was never again (afaik) repeated anywhere in any of Paul's supposedly "genuine" letters. It was a complete one-off remark.
2 that same James apparently wrote his own gospel where he expressed all sorts of beliefs about "the Christ" but afaik (according to those who have read that gospel), nowhere does that same James ever claim even to have ever met any such person as Jesus, let alone claim to have been his family brother.
3 iirc, in writing about that same meeting where he saw James with others who he describes as the "Pillars of the Church of God", Paul never mentions asking this James person, a single thing about his "brother Jesus". And that same James who he saw at that meeting apparently also never told Paul nor anyone else a single thing about ever being a family brother of Jesus. And remember here that we are talking about Paul having his entire life totally changed by this belief in Jesus, and yet when he meets the actual brother of Jesus, not one word is ever mentioned about any of them knowing or meeting Jesus at all.
4 on the contrary, what Paul did say in his letters was that those people he met as the "Pillars of the Church", inc. James, were claiming to believe in various different people as the messiah, and he rebukes them all for that, where he also tells them that he has no interest in what any of them say or believe about a messiah, and that his own belief is the one true understanding of the prophesised "Christ", because he says that none of those other leaders of the Church had told him anything about the Christ ... he specifically says "the gospel I preach came from no Man" and "nor was I taught it by anyone" ... so he is getting none of that Jesus belief from the Pillars of the Church inc. James, nothing from them at all, and he tells us that they each believed in a variety of deceased or otherwise absent or non-existent names as the one-time Christ upon the Earth.
5 the only sense in which Paul ever suggests that he and James and "500 others at once" and "the twelve" had ever "met" Jesus, was as a spiritual vision of Christ in the heavens. Nobody else, inc. Paul, ever claimed to have met or known Jesus in any other sense ... and that included James in his own gospel who afaik also never claimed to have met any human Jesus.
6 In the same letters, Paul makes it repeatedly clear that his knowledge of Jesus came to him through divine revelation from God, where he says "God was pleased to reveal his Son in me", and where he says that what was revealed to him was a new understanding of "Scripture" where he suddenly realised that the true meaning of ancient messiah prophecy was that "the Christ" (he uses that term so often that I wonder if the word/name "Yehoshua"/"Iesous"/Jesus was actually also yet another later interpolation?) had already descended to Earth from the heavens in order to prove to the faithful that the long-awaited (since at least 500BC) apocalypse of God's final judgement was now very close at hand.
7 the structure of that half sentence about James, is this - it says “other apostles saw I none” then there is a comma or "pause” and then it continues “save James the Lords brother”. That structure looks suspiciously like it may have originally just said “other apostles saw I none” … and then at a later date a Christian copyist decided that he should have also seen James at that time, and so he adds the words “save James” … and then either at that same time or at another later date another copyist decides that the letter should explain who “James” was, and he adds the final 3 words “the Lords brother”. But be careful here, because I am not presenting that as if it think it must have happened that way … I am just pointing out that it is constructed in the form of 3 separate afterthoughts, i.e. the first bit is just to say “other apostles saw I none”, but then someone decides that he should have also met James there, so as an afterthought he adds “save James”, as if to say “oh, and I forgot to mention that James was also there”, and after that there's another afterthought to explain who James was in case they did not know who he was and now he adds “the Lords brother” … I'm just pointing out that the structure of that half-sentence is in the form of a series of additional afterthoughts.
8 the reason to suspect so-called “interpolations” is because after nearly 2000 years, biblical scholars and others eventually realised that much of the writing about Jesus does contain what appear to be later interpolations where scribes who made copies at a later date, decided that certain parts of the original writing had to be changed either to remove something they now disagreed with or to add something which they had now decided should have been included in the original.
9 In that respect of interpolations and later copyists reproductions - we also have to keep in mind that the earliest copies that we actually have for any of Paul's letters most probably date from around 200AD, and not from anywhere near 50 to 60 AD as all biblical scholars like to imply. And that leaves about 150 years of constant copying and re-copying by scribes who were apparently in the habit of altering the original texts. So we really cannot rely on those copies known as P46 to be exactly what was originally written in the letters, and that includes not being at all confident that the ultra-brief half-sentence about James was originally there or in that same form of words.
10 you, along with everyone else who believes in a HJ, are quoting passages from the gospels and letters of the bible. But it is clear that none of the people who wrote those gospels and letters had ever known any such living person as Jesus. In all of that biblical writing, the authors only ever describe their belief in a supernatural being of the past that none of them had ever met. So at best, what evidence there is in the gospels and letters is only evidence of the writers belief in a supernatural deity that the writers had never known.
And please lets be clear - in all of that biblical writing, that is how Jesus was always described, ie as constantly supernatural.
The evidence contained in the bible is indeed evidence of something, but it is not evidence of a human Jesus ever known either to the writers or to anyone else. It is only evidence of how the biblical authors believed in a supernatural scion of Yahweh that none of them had ever met, seen, heard or otherwise known at all.