For what it’s worth I was raised to think that the brotherhood were those within the one cup fellowship. Whether or not one “brothers” a person had to do with his alleged doctrinal purity. Even though those in other segments of the Churches of Christ responded to the gospel “legitimately”, subsequent mistakes in thinking about certain issues nullified the brotherly relationship. I no longer believe in this position. In fact, I think it is anti-scriptural, unscriptural, and wrong. It is detrimental to the cause of Christ.
Also, for what it’s worth, this was not the position of the early Restorers. Their motivation was “to unite the Christians in all the sects” which indicates they thought of persons as brethren who did not belong to their particular sect. Were some of these people ignorant and lacking in knowledge? Were some of them mistaken in their thinking? Were some of them wrong in methods of obedience? The answer to all three of these questions is yes, but their mistakes did not nullify the family relationship. If perfection is required we are all doomed because who has made a perfect response to God?
Alexander Campbell once defined a Christian thusly: “Every one that believes in his heart that Jesus of Nazareth is the Messiah, the Son of God; repents of his sins, and obeys him in all things according to his measure of knowledge of his will.” I agree with this definition and this in turn determines brotherhood. It is not determined by sectarian interpretations relative to disputable issues as we have practiced in the Churches of Christ. How did John say to test for brotherhood, by asking where a person stood on interpretations of the New Covenant scriptures? No! By asking if he believes that Jesus is of God and had come in the flesh (1 John 4:1-3). If a person confesses such a belief and is living in obedience according to his knowledge commensurate with that belief, manifesting the Spirit of Christ, how can I say he is not my brother.
I realize this raises questions relative to such issues as baptism, particularly its mode, but as Alexander Campbell said when replying to the critic of his use of the term “Christian”: “It is the image of Christ the Christian looks for and loves and this does not consist in being exact in a few items, but in general devotion to the whole truth as far as known.” He had previously said, “I cannot, therefore, make any one duty the standard of Christian state or character, not even immersion into the name of the Father, of the Son, and of the Holy Spirit, and in my heart regard all that have been sprinkled in infancy without their knowledge and consent, as aliens from Christ and the well-grounded hope of heaven.”
The bottom line relative to answering this question is that the brotherhood is bigger than our faction and being brethren-in-error does not negate that relationship. We are all brethren-in-error.