>>279718022 (OP)Just because people don't know how to write it anymore i think.
I mean, people here used it a lot as reference but in Yosuga no Sora you have the sister route and some JP cultural references like Sora wanting to die drowning so she could be reborn as someone without Haruka blood, to destroy the highiest barrier in their love relationship, and you also can see and even if you hate incest you can't negate the reason for it, the social, emotional, familiar and even the reason for her own existence which is her brother love.
And there's too the fact they don't just fuck, promise to love each other being blood related and that's it, they face a lot of consequences for it, throw away friends, school, a future, everything in the name of their love while you have new stories like that russian girl anime where the sister just teases and nothing more, it is treated as a fetish joke of the author and not an taboo to explore deep in.
>Tl; Dr: It is just that people don't know how to write the taboo of incest anymore, probably because society changed too much for it to be viable again.