"all Jesus does in this passage is to call every Christian his brother. "
No. A herald announces that His Mother and bros are here and outside looking for Him. He uses the occasion of that announcement, to to say that all are His mother and brother. Why doesn't your analysis apply to the woman too, so that it means it was some woman and her cousins outside looking for Him?
"In fact, this is another reason why all the references to "brethren of Our Lord" are problematic."
There's no real problem. The bulk of the evidence says that it is more probable, that He had siblings and as I said, it not conclusive.
I'd like to see that probability calculation.
-A8
Yes, in that passage alone Christ suggests that his disciples are His brother, or sister, or mother, depending we assume, on sex and age. But the expansive use of "brother" to refer to spiritual kinship we find in many places.
My point is that the passage does not say anything about Christ's disagreeing with His mother or other kinsfolk. It is not them, but rather some unspecified listeners who think He had gone mad.
Just like with "brothers" being necessarily Mary's biological children, we can interpret this as a sign of a disagreement, but the text does not compel us to do so; it is a matter of interpreting the text.