obviously the subject of this BL is some kind of strange fetish, especially the one who wrote the novel, who juggled to hide the incestuous relationship of the two, it may be taboo but I have no problem talking about it and criticizing it, I've heard the police say that most rape and pedophilia happens in the home, it's not a subject to be treated so lightly.
incest noun sexual intercourse between closely related persons. the crime of sexual intercourse, cohabitation, or marriage between persons within the degrees of consanguinity or affinity wherein marriage is legally forbidden.https://www.dictionary.com/browse/incest
Incest (/ˈɪnsɛst/ IN-sest) is human sexual activity between family members or close relatives.[1][2] This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by affinity (marriage or stepfamily), adoption, or lineage. It is forbidden and considered immoral in most societies, and can lead to an increased risk of genetic disorders in children in case of pregnancy.https://en.wikipedia.org/wiki/Incest
of course one adopts the other child, spends years as father and brother, but they've always had carnal thoughts, you don't see anything wrong or strange in that.
I agree, I don't like this kind of story, I'm not Asian, I don't understand their view on the subject, but in my country today we value feelings, love more than just blood relations, so whether they have the same blood or not for me, it's an incestuous relationship, for everything you've described.
sexual intercourse between closely related persons.
the crime of sexual intercourse, cohabitation, or marriage between persons within the degrees of consanguinity or affinity wherein marriage is legally forbidden.https://www.dictionary.com/browse/incest