In reality, Tiffany and Taeyeon are friends with Seo Juhyun and her sister Hwanhee is really dead. Seo Juhyun is stuck in limbo because she's trying to deal with that loss by living in fantasy. Tiffany and Taeyeon kill Hwanhee, using that kind of pain to trigger Seo Juhyun memories. Hwanhee comes back from the dead because Seo Juhyun’s subconscious willed it serving as a kind of defense mechanism because it was too painful.

All writing/story credits to: peppermintmuffin.wordpress.com