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.
If you will pardon my play on words, we in the Gentile church have a strong perspective to take a prospective view of this passage. As you are probably aware, there has been scholarly debate as to whether this, and other passages, are prospective or merely addressed to the immediate audience. Given the fact that Christ uses the present tense in stating that all who hear and do the will of God are His mother, brothers, and sisters, rather than will be His mother, brothers, and sisters, can be taken to imply an immediate context. However, there is no doubt that it can, and should, be applied to all who hear and do the will of God regardless of time, both retrospectively and prospectively. Thus, Abraham, the friend of God, or David, the man after God's own heart, could be called brothers of Christ.
However, that does not negate the immediate context of the statement. As with many things, Christ takes an immediate situation (the presence of His mothers, brothers, and sisters) and takes their relationship to Him to show the closeness of relationship believers have to Him,
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.
While I agree entirely with you concerning these relationships, there was a distance between God, the holy One, and his people, Israel. Without a priest interceding for them they could not approach God. The prayers of the people could not be offered and accepted before God without a sacrifice made by the priest. This is in stark contrast to the relationship of the Christian to God, as shown in the book of the Hebrews. Today each Christian has boldness of access through Christ into the presence of God and is accorded a deeply personal, intimate relationship with Him.
The tutor served its purpose, but now, in Christ, we are no longer under the tutor but are heirs with Christ.
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.
I agree again here. The metaphor of mother, brothers, and sisters has its limits and there are, indeed, no discreet divisions of relationship with God, although we are called to differing relationships with each other in the Church - older women to be viewed as mothers, older men as fathers, contemporaries as brothers and sisters. However, Christ gave the metaphor to us and, as such, it is valid as long as it not overly extended or misapplied.
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).
At that point in His ministry, Jesus had not given any indication to His disciples or to anyone else that the offer of oikos/suyennis would be extended to all the nations, not just the Jews. Thus, the disciples were not concerned that our Father would include non-Jews, but that one could address God is such an intimate fashion. This was in radical contrast to the prayers of the day.
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.