I don't quite believe that getting rid of duke had anything to do with jealousy. Feels too uncharacteristic, too stereotypical. Especially since it was her idea Lenore should spend a lot of time with other people in the first place.
I think it's much more likely that Annebel Lee has finally realised that Lenore isn't cold blooded enough to make her plans work out as she keeps attaching herself with everyone who is nice to her, against Annabel's advice.
I don't think she's trying to get rid of duke out of jealousy, that seems way too short sighted for Annebel. She is getting rid of Lenore's attachment, knowing that when the time for escape comes, Lenore might ruin things with her stubborn loyalty to some pawn she meet only a couple weeks ago.
The whole thing might have been avoided if Lenore had told her about her chat with the raven instead of freaking out about the battle royal revelation. If Annebel was aware how they'll probably need a strong group once they're out of the academy, she might no longer be so opposed to rescuing anyone else.
