If you read it like a more traditional fairytale this chapter would make more sense. I think her mother's sigil was the equivalent of a selkie's seal skin, and the moment the villainess got it back, along with the prince breaking the engagement of his own accord, released her from all obligation...
Reminds me of that one passage from Roald Dahl, the one from Matilda: "If a person has ugly thoughts, it begins to show on the face. And when that person has ugly thoughts every day, every week, every year, the face gets uglier and uglier until you can hardly bear to look at it."
Essentially...