At the end of The Life of Pi,
Investigators think Pi's story is to unbelievable, so he tells them a version where all of the animals on the boat were actually all people, and the horror of the events is so much worse. As neither version can be proven, Pi asks them which version they prefer and they choose the more fantastical one with the animals.
I think the different versions of the brothers' story is kind of like that. What did the sister want to believe, what did she know to be true? Which was the story she could make peace with?
I had to google The Seven Ravens. Many aspects of the story made more sense after that.