Is that after she is already cursed?
Cause Jezera curses her, and then you keep her for a bit, and then she escapes.
there is 2 option when you capture the fort,
take helayna as your prize and she is belong to rowan.
or
let the twin have her.
letting the twin have her will cause her to escape no matter what (alexa will help her escape in case rowan doesn't help)
taking helayna as yours will give another route to train her as knight or the other things.