But this got me thinking if is it normal thing in siblings.
I know you're asking girls but I was talking to my sister and my friend at one point.
Usually this is seen in the undertones of their behavior in a show, and it almost always happens when two people become step-siblings in adolescence.
I've never heard about this happening to anyone I know - I personally think it is quite strange for you're Sister's to do this actually but yet again they are doing nothing illegal or really wrong.
The brother has spent most of his life with a crush on his step-sister and despises their parents for making them siblings.
Takeru heavily pursues Kayano, and naturally she gives in.