Love the maids as a Greek Chorus for this.
@karolyn
The Duchess seems too nice and too perfect.
She does have a believable mixture of "this kid could be an incredibly powerful asset - and it would weaken a rival noble house to take her" (she doesn't seem to know the full tale, but there's the chance she's at least heard rumors about the sacrifice ritual, since it's been happening for a thousand years, in which case she's realized poaching Elise would basically be the equivalent of stabbing that family in the gut. Even if she hasn't heard about the sacrifice ritual to empower the heir, she does know Elise can black out her whole castle, while her sister hasn't demonstrated anything on that level, so she's still getting the stronger daughter), and having a degree of empathy with Elise over the fact they have (technically, had) similar scars given to them by people they trusted.
On the more sketchy side, it's been mentioned that the Duchess has three sons, but no daughters, so adopting Elise (despite what Elise said about leaving the family when she reached 18) could potentially give the Duchess a bargaining chip she doesn't currently have for any political marriage alliance she wants to make with folks that have an heir, not an heiress, anywhere up to six years down the line (and six years is a long time).
It's not full-on nice & perfect, but there are enough logical reasons this would all seem like a good idea to the Duchess which aren't flat-out evil that I'll give her the benefit of the doubt. And my impression of the earlier scar scenes was that the Duchess absolutely saw a reflection of herself herself in Elise, and is (in addition to having entirely logical reasons for the adoption) motivated to some degree by what she sees as their similarities.
