The work around

I've researched the topic of incest in Japan a while ago, and while it's not clear, I think there are several elements that make it a popular theme in fiction (I'm very sorry for the long wall ; ;).
One, it seems is that incest between families was common until practically the 20th century. The strong rejection about incest is especially prevalent in Judeo-Christian cultures, so in places like Japan, where Christianity doesn't have such an influence, incest isn't as shocking as it is in the West. As far as I know incest isn't currently illegal in Japan. Another is that many of the authors of these works are only children and don't have siblings, plus a lot of otakus are isolated persons who don't interact with people. And that could be related to this other thing: because it's a culturally shy and facade-like society, your siblings/parents/uncles know you on a real, personal, and intimate level so that adds to that supposed attraction. Also, it doesn't help that they affectionately call people not related to them by blood "brother/sister/uncle," so the line is also blurred in that sense. And of course, being somewhat taboo, it gives it that touch of forbiddenness and fetishism/kink, which is so prevalent in pornography, both real and hentai.
Regarding adoption, I don't think (or want to believe) it has much influence on incest. Adoption between adults is common in Japan, even among heterosexual couples. It's especially common in companies that have only daughters and want a heir, so the family adopts the daughter's husband so he can have the family name, since otherwise it would be lost if she married in the standard way.

These are the things I could find related to this topic. It's nonetheless a cultural thing that provokes a literal culture shock to readers. Thank you for reading until the end (hope so (￣ε(#￣)Σ).