Spoilers.
no they are not. They are siblings. however Miyuki is artificially modified so completely unrelated to Tatsuya DNA-wise.
(⊙…⊙ )
Wait wha- i never expected that
But are both their sperm and egg came from the same ovary and testicle?? I dont get their dna alteration part in the novel
Genetically alter or experimented on? To be a perfect lock and key for miyuki and tatsuya
so turns out they r cousins huh... in a lot of cultures in asia, marriage among close cousins r allowed, although not encouraged. so it lessens the incest factor just a tiny bit, i guess...
still, not my fave story though. the main characters r so strong & perfect, that it became a turn off.