Lily had deceived Rose all along. That day at the cemetery, she was the one who had organized the trap. Dylan had really been on guard duty and, as agreed, signaled Lily upon Rose's arrival. Her cousin had pretended to be on her side and insisted on not going to Knockturn Alley only to have the opposite effect and push Rose there, into the Dark Wizards" lair. The same had happened with Trevor's letter, ordered by her just to incite her cousin to come back to school. Lily was the only one who knew her well enough to create such perfect traps for her.