Why do I have a feeling that the little sister is someone from his past life as well? If she turns out to be a reincarnation of the kohai he saved, I wouldn't be surprised.
How far back does this family tradition go? If its more than a few generations then the main could conclude that something about the bloodline or the world keeps incest babies from turning cross eyed. It would still be strange to someone from an earth culture though.
let me guess dude. That girl he said from his previous life studied the rest of her life in black magic and somehow managed to reincarnate as his younger sister because of her guilt and love for him. And she'll eventually gain her memories of her previous life later.
@Aichan probably a mistranslaion of what I'm assuming was in katakana "シャーマン/shaaman", which can be interpreted as either "sherman" or "shaman" (correct title is likely "reincarnation of shaman")