I can’t find where scripture says explicitly that Jews and Christains are “Brothers.” A search of the RSV for the word brother is
here. But the concept of your question sounds familiar somehow.
I think the idea of Jews and Christians as brothers however, is not that far from reality as they earliest Christain’s would have seen it. In the earliest Church, immediately after Jesus, Jewish converts to Christianity did not consider themselves to be non-Jewish. If asked, they would have indentified themselves first as a Jew, yet as a Jew who had realized that the Messiah had indeed come.
The Jews who did not accept Christ as the Messiah and the Jews who did were not seperated like they are now. The New Testament had not yet been written and codified and Jewish converts to Christianity still went to synagogue with their non-Christian Jewish brothers.
This started to change after AD 70 when Jerusalem and the temple was destroyed and Jews were slaughtered. Around AD 85-90, Jews start expelling Christians from the synagogues because, 1) many Jewish converts to Christainity would not practice strict adherance to the mosaic law, 2) some blame was placed on Christians for the destruction of the temple and of Jerusalem, 3) persecutions of the Christians put those who associated with Christians in great danger. Full seperation of Jews and Christians took several hundred years.
So to the earliest Jewish converts to Christianity, Jews who had not accepted that Christ was the messiah may not have been referred to as “Brother”, but would most certainly have been seen as fellow members of God’s chosen people. And it is important to remember that God himself refers to the nation of Israel collectively as “My chosen son” (singular).
-Tim-