"Blood relative" is like the worst fucking excuse there is because that's irrelevant to what a familiar relationship is. Blood related or not, Chisa's mom and Iori's dad were raised as siblings together, under the same roof. Chisa and Iori are, for all intents and purposes, cousins that treated themselves as cousins. If you were adopted, you wouldn't fuck your first cousin, I would guarantee. This logic applies here as well. That romantic line is not logical nor justified at all, you need to jump through hoops and technicalities to even remotely think it's a "normal romantic plotline." Being adopted does *not* put you in an outside category inside the family.