@mirmirmir
Incest requires Blood-relation, Inseki only needs a legal familial relationship. Also Incest is more taboo than Inseki due to Inseki not having the risk of birth-defects and thus it makes sense to separate the terms.
I'm not expecting the girl would be this chill knowing that she did "flop-flop" stuff with his brother and also there is no further explanation like is that her first time? Well knowing that she also drunk and seduced his brother maybe she doesn't give a damn about it because that's totally her fault, anyway i can already imagining what would happen next
Shoujo genre really have a big different when it comes to romance