I've actually thought about this some, lol. I will cast the net wider and say incest in general is a common theme. I have never asked a japanese person this question so this is total conjecture, but my guess is it has something to do with their "inner" and "outer" circle culture. In japanese culture it seems like it is more difficult to get close to people as adults, and there is a degree of intimacy that is impossible to reach unless you knew someone as a child. So I guess that's how falling in love with your sibling/relative can become an attractive alternative????? (I made a strange face as i typed that, lol)
Also, I would watch Kattun do just about anything, lol. Maybe they're not really siblings and they're just close childhood friends? ...