Shisui = Loulan is my theory as well.

• Loulan is extremely private, not really leaving her pavilion unless she has to. Also, hardly talks at all in public
• Shisui is oddly knowledgeable about tons of stuff that a normal servant girl wouldn’t be (Maomao even compared her to Gyokuyou this episode)
• Shisui deftly avoided coming face to face with Lishu this episode, perhaps she would be too easily recognized
• Mamao commented in the ghost stories episode about how good of an actress Shisui is
• This episode they commented on how different her body looks when she is undressed

I think its very clear Shisui is hiding something and she didn’t really enter the story at all until Loulan came to the rear palace. So, that is my theory of the case right now.