that is a point, which I can't grasp at all, I can understand and respect them, your all points of view, some like the one of DS or GOA I can deeply comprehend, impling that I can see them trough their prism of values also respectable.
For myself, and my own set of values and believes, that what Kaija did is not so terrible, yes, she lied, yes, she did not tell us the whole truth, and still hasn't told us everything.
The betrayal, if we are fair, is on both sides (depending how you play the MC) and the relationship, is on a braking point, but not so because of Kaijas behavior, more about the conflict what the MC has with himself, he feels as they are drifting apart, (no wonder he lives in a mundane world, and Kaija lives in a totally different world with magic, monsters and fairytale creatures, hidden dangers and threats, which are real as well.
MC is overprotected,(and acts somtimes as a selfish prick not much hero like) and was kept apart from all that what is going on as well as his sister, with not bad intentions, their parents wanted to give them, the chance to live without all that trouble and violence which has this paralell world, they all did what they did, with the best of intentions, and not with premeditation and alevosia.
Kaija has her own conflict with all that, she is a pawn in a game to which she agreed in order to revenge or to learn what happend with her parents.
You can like her or not but I never had the Impresion that she is a vile perfid liar, and she is pretty assertive with what she feels, has high and somewhat pathetic Ideals at times, but she is not that monster which she is painted somtimes over here.
And that is what I can't put my mind around, maybe it is not in my personality.
(Of course is fun to hold against, all Kaija detractors, and I whish Torin sums up in the struggle
because together we are unberable
)