Investigating a minor trivial possible crime, help?

Discuss it with your parents and let them handle it. If your cousin is in possession of a game her parents didn't buy, you probably have your culprit. However, if the game is hidden, it's anyone's guess where it disappeared to. Should that be the case, you can't prove your cousin took it. Perhaps the other cousin will snoop around for it. Not an ideal situation, but sometimes it works.