and only pretended to be her sister because she really wanted to be the one who went
Now you're just moving the goalposts. If she really wanted to go that badly, why not let their father know? Why did she have to lie and trick to increase his burden, ultimately leaving her sister by herself much sooner?
That right there is the biggest part of my issue. The road to Hell is paved with good intentions, you know?
No, that's a fact. The original plan would have Ravina still in town. Ravina has no issue with going with the pirates, heck, she doesn't even seem to hate them or anything (I'm not saying she likes them). So assuming we go with YOUR assumption and Pig Captain would go for either sister, the problem would have solved itself if it was only one of them. Which you said was my assumption, but is shown to be actual fact. After all, Ruruna's life being in danger is the only problem Ravina has with the whole situation.
EDIT: If the pirates had no sister to take, they'd be treated like the other cities in the region, no? I agree that the Sage wasn't "right" but having both sisters does indeed increase the burden on the town because of this unresolved situation with their captain. Since nobody noticed outside of the region yet, it would seem the territorry is being "harvested" by the pirates moreso than being "plundered." They'd notice eventually, and then the problem would be solved.
So despite everything I said, I have to agree that considering the pirates the calamity is rash. I won't move on how pointless it was for Ravina to take her sister's place, though.