If someone truly believes in God and has committed their life to Jesus Christ, then their life will never be the same. The Bible says, "Therefore, if anyone is in Christ, he is a new creation; the old has gone, the new has come! All this is from God" (2 Corinthians 5:17-18).
This doesn't mean they will be perfect; only in heaven will we be completely free of sin. But it does mean they won't live the same way they did before they came to Christ. Their goal will be to please God instead of themselves, and to live their lives for His glory. They will be sorry when they sin, and they'll want to confess it and seek the Holy Spirit's help to overcome sin's grip on their lives.
Only God knows your brother's heart and whether or not he has honestly given his life to Jesus Christ. Tragically, many people today are outwardly Christians and yet their faith is only superficial, and they have never truly committed their lives to Christ. This is one reason why our greatest need in the church is to pray for spiritual revival, asking God touch the hearts of those among us who are only nominally Christian.
Pray for your brother, and ask God to open the door for you to talk with him humbly and yet openly. And don't neglect your own spiritual needs. Is Christ the Lord of your life?