1 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father’s wife.
2 And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you.
3 For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed,
4 In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ,
5 To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
6 Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?
7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.
9 I wrote unto you in an epistle not to company with fornicators:
10 Yet not altogether with the fornicators of this world, or with the covetous, or extortioners, or with idolaters; for then must ye needs go out of the world.
11 But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer, or a drunkard, or an extortioner; with such an one no not to eat.
12 For what have I to do to judge them also that are without? do not ye judge them that are within?
13 But them that are without God judgeth. Therefore put away from among yourselves that wicked person.— KJV, 1 Corinthian 5
A 1cor5brother is someone that is continuously and habitually into one or more of the 6 sins mentioned in verse 11. The sins are fornicator, covetous, idolater, railer, drunkard and extortioner. So living a life of continuous engaging in one or more of these sins.
Saved or lost is irrelevant here. What is important is that this man is called a brother. Indicating that he claims to be a believer in the lord Jesus Christ. (Or possibly that there is some close family relation to him, see definition of brethren).
How should we deal with a 1cor5brother
The way to deal with these people is described in verse 11 and 13. We are not to eat with them and we are to put them away from among us.
Notice also that it cannot be a sister because it says brother.