We had a couple who were members until we learned that their marriage was unscriptural. They refused to repent. Are they considered "brethren?"
"And if anyone does not obey our word in this epistle, note that person and do not keep company with him, that he may be ashamed. Yet do not count him as an enemy, but admonish him as a brother" (II Thessalonians 3:14-15).
They are fallen brethren, but still a brother and sister. That means that if they ever are restored, they will not need to be baptized again because they are already are a part of the children of God.
Yet, while they are in this state, all the relationships that make it special to be a child of God among many are withdrawn. They are no different than other sinners in the world. "And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a heathen and a tax collector" (Matthew 18:17). Thus, we are polite, as we would be to anyone else in the world. We are insistent that they turn their lives over to Christ, but it is the same as we treat everyone else in the world. But we don't make them feel that their current state is acceptable. "I wrote to you in my epistle not to keep company with sexually immoral people. Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world. But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner--not even to eat with such a person" (I Corinthians 5:9-11). Notice that even in this last verse the sinner is still called a "brother." In fact, it is because he is a brother in sin that he is treated in this manner.