Hello Anon! Well, the last time we truly saw Lizzie was in ch116/117 and…
…She seemed more worried about stopping Seb from going to the labs rather than fighting against a real demon, so no, I don’t think that she knew at the time.
Of course, there is still a possibility that ever since ch117 Bravat or real!Ciel told her about Seb’s true nature (since Bravat definitely knows and real!Ciel seems to know as well that Seb is the demon who ate his soul 4 years ago), however, assuming that Lizzie is most likely seen as a pawn to hurt our!Ciel [x]…
…it would be wiser of Bravat and co not to tell her, in my opinion.
Lizzie is currently conflicted about making the right choice between her duty (real!Ciel) and her feelings (our!Ciel) [x][x] so, should Bravat tell her that our!Ciel sold his soul to a demon, she would probably be sadder at the idea of losing him and it could push her to disregard her duty in favor of her feelings.
It might not appear blatant, but I’m guessing that arguing that real!Ciel was in poor “health” is probably the argument that won Lizzie over when it comes to her running away from home:
She decided that she would be here for him, because he was in dire straits, and then she stayed because he is her rightful fiancé.
Lizzie is most likely the ace of Bravat’s side, because she definitely is the person our!Ciel cares the most about (it’s even how Bravat referred to her), which means that they can’t afford to have her changing sides.
So, not telling Lizzie about Seb to keep her thinking that real!Ciel needs her the most (on top of her doing her duty) is the most logical.
Maybe she’ll know before the end of the arc though if, for instance, she changes sides and a very angry real!Ciel drops another bomb on her: the boy she loves not only lied about his identity but he also sold his soul to a demon and won’t live old.
TL;DR no, I don’t think she knows for now. 🙂
I hope it helps, have a nice weekend Anon!
midnight-in-town 