"Yes, in that passage alone Christ suggests that his disciples are His brother, or sister, or mother, depending we assume, on sex and age."
No. He suggests all are His bros, sisters, maothers ect. That was in response to the announcement, "your mother and brothers are outside looking for you".
"But the expansive use of "brother" to refer to spiritual kinship we find in many places."
Sure, but not in this passage, or John 7. In this passage, the distinction is made between his bros, as in related and close to Mary and all others.
" My point is that the passage does not say anything about Christ's disagreeing with His mother or other kinsfolk.
I didn't say He did. I have no idea where you got this from.
Re: why he didn't refer to John the Baptist as "brother John"?
"1. The recorded speech of one of them to the other tends to be coached in lofty terms: "I must decrease for He must increase"; "so it becometh us to fulfill all justice". This precluded "brother" as an affectionate term."
Irrelevant. THe term bro has no connotations of stature.
"2. The expansive use of "brother" is common when several relatives are addressed collectively. When just one person is addressed, a more precise term, such as "cousin" would be typically used."
No. The term may apply to cousin. It is not a term that refers to generic relatives.
" 3. John was renowned in his own right, while James a.o. were only known in relation to Jesus."
Both John and James were only "renowned", because of the existance of Jesus.
Read v. 35, "For whosoever shall do the will of God, he is my brother, and my sister, and mother."
not in this passage, or John 7. In this passage, the distinction is made between his bros, as in related and close to Mary and all others.
In John 7, I agree, the likely reference is to relatives.
The term may apply to cousin.
OK, but I don't think Jesus referred to John the Baptist as a relative in any particular way. Why? I offered some ideas, but the simple answer is, we don't know.