@rei893 You have to be related by blood for it to be incest. The whole reason we have the taboo of incest is because of the complications(birth defects) it causes in offspring. You wouldn't have those complications if step siblings have offspring so no incest.
@justforthelulz Oh! So that's what that tag means... *jots down note for "research" later..." I see, I see...
hmm…
(1) If I say "incest", some people in some contexts will think it's about genetic relation and other people in other contexts will think it's about familial relation. For practical purposes it can mean either. If I'm talking to a biologist, I only mean the former; but if we're talking about romance manga, both kinds of incest will trigger that sweet, sweet, doki-doki anxiety that we crave.
(2) Sure one reason for the taboo is birth defects, but you can be in a heterosexual relationship without having children. The other taboo-ness comes from the same source as eg teacher-student relations; being siblings, they live together, and otherwise have a lot of connections with each other that aren't trivial to escape (cutting off a family member is a lot harder than cutting off a normal ex). There is also usually an age difference and other power imbalances in an incestuous relationship, though that doesn't apply here, much to Amane's chagrin.
"inseki" is an interesting term; the fact that it exists… actually isn't it just the same as "step-" or "in-law"? How does Japanese differentiate the two?