Hi Thekla,
You certainly make some interesting points here. As you noted, the Bible is replete with descriptions of relationships with God. In the Old Testament it was very much a legalistic type of relationship. God was the creator of Adam (man) and therefore the father of all mankind. He was also the father of Abraham, Isaac, and the patriarchs. However, the concept of a personal. intimate relationshio with God as a loving Father was a very radical concept brought to the theological table by Jesus Christ. The Pharisees were horrified that He, being a man, could call God His Father. Not only that, but that He taught His disciples to pray, Our Father . . . Christ very much drew back the veil of separation between man and God so that He could, in truth, relate to believers as He did His own mother and brothers and sisters. To distance that close relationship as being a relationship between cousins or kinsmen brings it closer to the Old Testament mentality. I believe that Christ intentionally meant brothers and sisters and used his own brothers and sisters as well as his mother for his analogy.
the rhetorical turn employed by Christ in the use of "adelphos" narrows its future (towards the spiritual, post-resurrection, as attested in the epistles) use, not its use contemporary to the event from which its is drawn; it is prospective, not retrospectively narrow.
As for the OT sense of relationship, please excuse me for disagreeing. The relationship with God experienced by Adam, Moses and others was deeply personal. The Law, imo, was formative, developmental in skopos, and relational - it "fenced off" the Jews as the people of A God (as opposed to gods), and served as an instructive and developmental tool (much as happens in the parent child relationship). As Paul notes (as in our previous discussion on the matter), the Law is the tutor, not the teacher.
Note also that in the OT, the Law created the "suyenis" of God (tribe, kin, etc), a meaning included in the broad definition of adelphos. "And they said, believe (trust) on the Lord Jesus Christ, and thou shall be saved, and thy house." Acts 16:31 Abraham also had a deeply personal relationship with God; it was through his pleas with God that Lot (his adelphos/nephew LXX) was spared from the destruction. In the NT (Acts, above) we see the notion of oikos/suyenis/adelphos maintained; in Christ, we have no discreet relationships (mother,and all meanings of adelphos), but all are offered sonship in Christ - spiritually ALL relationships are collapsed into one term: SONSHIP through adoption.
And the mystery that Paul teaches, is that the oikos/suyennis is now extended to ALL the ethnoi/nations, not just the Jews.
The scandal of "Our Father" is both that 'our' extends beyond the Jews, and that the Father is not 'mine' (hence particularized relational terms like mother collapse in the face of sonship through Christ realized in the oikos of His body).
In short, in the passage under discussion Christ turns all relationships (broad adelphos, mother, etc) into sonship by adoption through Himself.
As it is said, God has no grandchildren, just children.