For those who want to know how twins work:
There are two types of twins which are identical and fraternal.
Identical twins come from the same zygote so they have the same set of genes. Which means if they experience the same kind of life then you can expect them to be actually identical. Biologically they are the same person.
If they have children with the same person then their children would be siblings and if they have children with different partners then their children will be half-siblings.
Fraternal means they were born together in the same womb so they don't have to be necessarily identical. Biologically they are just siblings.
It's not impossible for fraternal twins or siblings to look identical but it is low considering that statistically a couple could have 64 trillion possible different children considering normal combinations of chromosomes alone without any mutations or other variations. (The real number is waaayyy bigger)
The whole process involving genetics, gestation and living life is very complex and science is still learning.
Not sure if this was a mistake or done intentionally by the author or translator but for me whether they are identical or fraternal depends upon their natural hair colour. If both of them have naturally black hair then I'll just assume they're identical twins unless we are shown some other characteristic which is different for both of them (for example, blood group which is quite common in drama shows but that would make them even more different).
This is to keep the lore simpler in my head.
So far I've only read the first three(3) chapters and the only clues we have is that: they look identical except for hair colour and have slight personality differences. Plus the younger sibling Fuuka said something which kinda implies that she always had dark hair as if her sister didn't. So maybe they really are just fraternal twins that look very identical except for hair colour.
It's also possible that one of them had a slightly different experience (like some kind of birth defect) from the other after they split in the womb which could make them genetically identical but show a different phenotype. Or maybe she just always keeps her hair dyed.
Right now they look different enough and since they're at different schools that should create more difference. Ain't no way somebody who's really close to them is gonna be unable to tell them apart.
What I'm trying to say is that either way it can work and let's be real, you're probably here just to see a threesome or some weird family drama. Or maybe you are one of the exceptional few who saw the cover and waltzed in expecting yuri incest? Pro tip: you can tell it's not yuri because the two girls are looking at a third person camera and not each other.
Hope this was helpful.
