@The_FallenAngel You're pretty ignorant about the psychology behind incest, huh. Yeah, you're fully wrong about everything you just said there. Sexual relations between close-age siblings ends up being pretty goddamn destructive in practice the same as when it happens between family members with higher age difference. On the off chance it isn't, whether it's between siblings, parents or otherwise, and there's no wrong actually being done, then there isn't anything wrong with it, triply so when there's not chance of pregnancy. It's the difference between standard gay sex and gay rape.
The believability argument is even dumber here, because not only is this not meant to be particularly believable, but the fact that you think it got unbelievable just now speaks quite a bit about your lack of understanding of how these things work. It's not believable to have dudes fighting orcs and dragons, but we read it all the same, and it's been like that with this manga as well, only it was about a mom and a daughter happily slamming it down together, and just like the dragon thing, it's an appealing fantasy to some and not to others.
People who don't like dragonslaying don't read manga about it, so let me ask you again: Why in the fuck are you even here? You seriously thought this was going to have a documentarian style, even after all these chapters? How are you this blind to the author's preferences and the audience they're writing for and how little you belong here? Do you enjoy getting angry about nonharmful things, in fiction no less, just because they go against your extremely subjective sensibilities?