Neighbor is your literal neighbors. Those within your community and who you cross paths with regularly. Same with your enemies.

There is a sense in which the application of neighbor and enemy extends more broadly, like on polity and nation-state levels, but the post war consensus universalized everything into abstraction. We are not called primarily to love our neighbor "humanity" in the abstract but those in the locales in which we are embodied and rooted.

Brother is any Christian believer but again the emphasis is on local church bodies and community life. I share a bond of brotherhood and fellowship with all God's people across time and space but I can only express the "one another" commands of the NT within my own local context.

As for your comments about blacks and Jews, what specifically do you have in mind? The statement is speaking about two pretty diverse people groups in a generalized way.

Christianity redeems and purifies all peoples and cultures. Each peoples has their own particular forms of degenerative sins. Some are arguably more destructive than others but none are beyond the redemptive grace of God and "dealing with them" usually comes down to specifics.

Also worth noting that the Church and a righteous magistrate have their particular spheres and duties in applying and upholding God's righteous standards. So the Church has a way of "dealing with" degeneracy that is distinct from the State.