Original: As Hannah, she must keep her fans unaware that she is really a normal kid, and as Miley, she must keep her friends unaware that she is really Hannah Montana. In interviews, Miley Cyrus spoke of the implausibility of being able to keep a secret such as Miley and her family do in the show in a world of intense media scrutiny.
Simplified: It has been shown that Miley's friends and most members of the Stewart family know that Miley is Hannah Montana.