@s0m30n3_a51an
I think you've got it backwards, if I'm right it's not that Eli suddenly became a good girl, it's just that she misunderstands many things in life and bases her actions on those misunderstandings. for one, Eli doesn't understand just how much her sister has been hurt by the family and thus thinks that Leslie just wants to leave, she doesn't realize that Leslie wants revenge. two, as far as I am aware, Eli doesn't know the whole incident that got Leslie burned was meant to be a human sacrifice ritual to transfer Leslie's at the time dormant power into HER. and more specific to this event (IF that was Eli and not the maid who overheard the master the night before) is that Eli thinks that if Leslie succeeds in hiding her powers and leaves then all Eli's problems will go away and she can just be the next family head like she's always been told she would be whereas if daddy finds out exactly how powerful Leslie really is he might just choose to drop Eli and give Leslie everything instead. (not sure which way he'd go on that honestly, but if Leslie is as powerful as I think she is, that would've definitely have been the best choice possible at a point before certain metaphorical bridges were burned...a bit too literally on the burned part)
Now, I'm looking forward to the marquis' face when he learns just how wrong things went, especially since this event will almost certainly move up the adoption time-frame, leaving him near certain Leslie has high level powers and without her there anymore to raise/sacrifice.