You're right, of course; only God knows someone's heart, and only He has the right to determine their eternal destiny. Jesus warned, "Do not judge, or you too will be judged" (Matthew 7:1).
How should we react to those who don't want anything to do with God and reject His ways of living -- and even urge others to do the same? On one hand, we have a right to be upset at what they're doing, and to do whatever we can to stand for righteousness and resist evil. When Jesus saw people brazenly turning God's House into a place to make money, He drove them out (see John 2:14-16).
But on the other hand, we should pray for those who don't know God or want anything to do with Him. God loves them -- and so should we. He loved them so much that He sent His only Son, Jesus Christ, into the world to give His life for them. Jesus wept over those who had rejected Him, and from the cross He prayed, "Father, forgive them, for they do not know what they are doing" (Luke 23:34).
Ask God to help you (and your cousin) to see those around you the way He sees them -- not in anger, but in love. Then ask Him to help you share with them the good news of Christ's love and forgiveness.