The Target Stone: Jon Becomes Zoe's Twin Brother

Zoe thought that Jon would just become a Goth like her (that way they'd be closer), but instead Jon became her twin brother, essentially making them the same age. This way, he was more like Zoe, in one particular way. In the next room, Jon was still panicking. He *had* to get the stone back. It was obvious that someone had stolen it. But who?