Chapter 6:
Let me quote myself here :
I actually wanted to pause reading the chapter halfway, and come here to post that I think they’re siblings, because why would that place they had never been in be important for both of them? But then it was explained anyway. Anyway, I don’t think we’re necessarily dealing with incest. I think that’s why the woman is so cool about the man having a girlfriend. They were strangely close for adult siblings, but maybe they found each other later in life? I’m sure we’ll find out more soon.
By the way, the place they mention seems to be 白神山地. I don’t know why books avoid explicitly mentioning place names so much.