@Forikorder @kavinh
Ya'll are missing the point. Pig-man literally said killing people is a waste of life, so if they just gave him what he wanted he'd keep them hostage regardless. Besides, assuming the sage predicted correctly (which it seems they did), The best way for them to avoid the current situation without killing both sisters would be for one of them to leave.
So the father and whoever else decided on the compromise that the sister with the reasonable personality would leave to become a nun.
But Ravina, being the unreasonable rulebreaker she is, took her sister's place at the eleventh hour. Causing her father to suffer MORE because not only did he fail at recognizing his own daughter, he probably couldn't help thinking that she would suffer much more than her sister who was naturally quiet. This caused his subsequent illness and death. It was her fault because it was her decision that aggravated her father's worries, ultimately killing him.
To top it off, she came back: contributing to the current situation. If she stayed away, her sister would have likely stayed below pighead's radar; if she had let things play out the way their father planned, her sister would have likely stayed a nun and wouldn't come back to the village, so she wouldn't have to fear being killed by some crazy pig. Not to mention their father might still be alive, but that is purely conjecture.
And that's not even taking into account the fact that in all likelyhood she just ran away from the convent as soon as she got the chance, causing trouble for everyone in the process. Wasn't she an adventurer, not a nun? I'm a little fuzzy on that part, and can't be bothered to check.
My point is, she disobeyed their father for the sake of being a rebel, killing him in the process and putting her sister's life in danger years later. If you say this is all the fault of the sage for communicating a vague premonition, then you're only partly correct. If she had played along, her sister's life wouldn't be at risk, unlike if the sage had said nothing at all.