So you're saying a 15 years old boy would be prosecuted as a child molester for having a relationship with a 14 years old girl? Boy, things in the US can be really uptight.
Now, for the issue of incest I agree with Adug, they both know is wrong but they can't help it. Rika is full of jealously and admiration for his brother which may be the reason shes trying to get his attention. His brother on the other hand, is a horny pre-teen who saw boobs and now he's even hornier...
Legally, if both parties are minors, it's not child molestation, but abuse or juvenile sexual assault or rape, depending on circumstances. Japan's views of sex being what they are, 2 minors 'playing doctor' is just juvenile exploration/experimentation. Between siblings, the culture is somewhat schizo...cultural mores from old/ancient Japan were much more forgiving of incest--indeed, I'd seen some stories where it appeared to be expected that the kids slept together, so that they had some experience by the time they were married. Modern Japan is......well, as you see.
I like that they author shown how conflicted the brother is about actually *doing* anything with Rika. Voyeurism, though--pretty nearly everybody I've ever known took a peek when the opportunity presented itself. Who hasn't snuck peeks at old National Geographics or Cosmopolitan mags for the boobs?. The argument here about prurient content isn't about the content, but about Authority telling you what they think you should be seeing, reading, thinking, doing.....so will the Japanese be docile before Authority, or play the same Democracy games those Authorities have played, to get back what was taken from them?