Christians aren't perfect. Every Christian sins, and every day those of us who are Christians need God's forgiveness and grace. I often think of the Bible's words: "If we claim we have not sinned, we make him (God) out to be a liar" (1 John 1:10).
But when someone claims to be a follower of Jesus and yet makes no effort to obey God and shows no evidence of a changed life, they may be deceiving themselves. The Bible is clear: When we know God, our lives will be different. Yes, our old sinful nature still lives within us, but God also lives within us by His Holy Spirit, and we won't be content to live the way we once did. Jesus said, "Thus, by their fruit you will recognize them" (Matthew 7:20).
Only God knows your cousin's heart, of course, and whether or not he has sincerely trusted Jesus Christ as his Savior and Lord. But one of Satan's most dangerous tricks is to deceive us into thinking we belong to Christ and are saved, when in reality we aren't. Judas claimed to be one of Jesus' followers, but he turned against Him and was lost, because he didn't truly believe.
Pray for your cousin, that he will face his need for Christ's forgiveness and new life. Ask God also to give you an opportunity to confront him with the spiritual danger he's facing. And pray also for yourself, that you will stay close to Christ and be faithful to Him the rest of your life.