I would agree with alatir's advice... it is absolutely unbiblical to say that just because 2 people had sex, even for the first time, that they are married in God's eyes... you just will not find evidence of this in the Scriptures, so your brother is involved in serious sexual sin and if you love him, you will confront him... secondly that you ought to confront him is itself a biblical principle... Luke 17:3 (ESV) Pay attention to yourselves! If your brother sins, rebuke him, and if he repents, forgive him," Matthew 18:15-17 (NIV) 15 "If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over.
16 But if he will not listen, take one or two others along, so that 'every matter may be established by the testimony of two or three witnesses.' 17 If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat him as you would a pagan or a tax collector."
Remember that we ARE to make such judgments concerning how others live their lives, and this is particularly true of others claiming to be Christians. And even more specifically, this is the case for sexual sins. Paul had a member of the Corinthian church excommunicated because of his sexual sins, but what I am pointing out in particular in this situation is that you not only may judge, but you need to judge. Paul says, for the person involved in unrepentant sexual sins:
1 Corinthians 5:5, 9-13 (ESV) you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.... 9 I wrote to you in my letter not to associate with sexually immoral people 10 not at all meaning the sexually immoral of this world, or the greedy and swindlers, or idolaters, since then you would need to go out of the world. 11 But now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reviler, drunkard, or swindlernot even to eat with such a one. 12 For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? 13 God judges those outside. Purge the evil person from among you.
So you need to confront him, but do so in humility and love, knowing that you are a sinner too. ...
blessings,
ken