How will Lily Potter keep her secret from Harry she was told specifically by Dumbledor not to tell him who she was. (She was told to keep a down low because she was the person they saved harry's life for so long!) She had always bean their know one knew when she was 6 she was enrolled at Hogwarts and sorted in all houses so she basically lived with dumbledor so she would when ever Harry started Hogwarts, she would just follow her twin around and when he turned around she would disappear. ALL CREDIT TO J.K ROWLING I ONLY MADE UP LILY!