This 100% insane to even suggest but... what if Sade is a time traveller and Duke is her lost love
1) Duke's curse effectively prevents any living thing from laying their hands on him, such as other women. But it also protects him.
2) Alice's mother was cursed into a magic coma, it is possible Sade intended to put Alice into a coma as she was getting close to Duke despite his curse, but her mother got in the way
3) She hates Daleth and ruins her face, because Daleth is actually the original Sade of this timeline, not her twin, and thus is also destined to fall in love with Duke
(This distortion of the timeline makes Daleth super insecure and she ends up falling for Duke's brother instead)
4) The "everyone was against our love" is because she's 150 years old (and counting) who preys on what is still a little boy when all the cursing and stuff is going down