First off call it like it is. This doesn't take Charles Spurgeon or Ravenhill to answer this question, you're brother is living in sin if not lost as well as his girlfriend. The Word teaches that if we are Christians and believe contrary to something God teaches as true He will correct us. I meditated over this thought myself years ago about marriage vs sex. Biblically, one man + one woman + sex = one flesh. Sex was designed as a blessing meant only in the context of marriage, anything outside of that context is wicked and sexually immoral and God hates it.
1 Corinthians 6:9-11 "Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God. Such WERE some of you; but you were washed..."
Galatians 5:19-24 "Now the deeds of the flesh are evident, which are: sexual immorality, impurity, sensuality, idolatry, sorcery...just as I have forewarned you, that those who practices such things will not inherit the kingdom of God..."
This is what scripture says of it. Goodness sakes, it is at the very beginnnig of the Bible that ONE MAN, shall leave his family and cleave to ONE WOMAN, and they will become ONE FLESH. You can't be one flesh with one woman or man if you have slept with more than one person. I understand some people did this before salvation, and that even some true Christians have fallen to this themselves. God can restore spiritually what the flesh devours. Take the heid of Paul in 1 Thessalonians 4:3 that you abstain and flee from this sin.
As for the ceremony question, it was ordained by God for man and woman to be wed. I have not read throughout the OT like leviticus to tell you that it is written to have a marriage ceremony. But no where in scripture do you see a man and a woman sneaking off to have sex ordained or blessed by God. Based on tradition in all cultures it is easy to assume that marriage ceremonies were established and upheld by all people. God ordains marriage the right way, the way that glorifies Him in purity and purpose. If ya just go around and have sex with the person you are in a relationship you are not only breaking the purpose He has for sex, but you are destroying His name and being just like the world. Don't be deceived by the sinful mind.
As for being angry with your brother, that is ok. Remember first Jesus wept over Jerusalem before He tore into the temple and started turning tables over. Be broken over his sin the way God is, but also have enough back bone to turn tables over. Marriage and sex are a God ordained thing, He will not let those who treat it flippantly to go undealt with. Marriage and family are God established and even said by some if not most to be 2nd most important thing in life other than salvation.
Read Hebrews 13:4