I don't understand why her father didn't tell her about his agreement with the duke ( that as long as she annuls the engagement on her own, the duke has to comply to it) ?
Since it makes no sense in my eyes not to tell her, because she thinks she can't annul the engagement on her own, only the duke is able to.
While not knowing that she is allowed to bring up the annulment, she will force herself with the engagement, because she doesn't want her family to offend the duke...
Thanks for the update!