I don't understand

According to this drama's book, a scarred back girl isn't supposed to fall in love, am I missing something? Sure, it's a bit repulsive but I don't think it goes to the extent where she can't be in a relationship, that would absurd.

I also don't think that Hiroto and Chiyoko are blood related. Probably step siblings and it has to be that because incest is something I can never tolerate.

Japanese de Laclos?

Here's my theory: Hiroto and Chiyoko are not blood related, but children of two people who remarried. So Hiroto is biological son of the husband and Chiyoko the biological daughter of the wife. They are step siblings by marriage.
And their tense relationship is very reminiscent of Cruel Intentions and the whole de Laclos dynamic.

If this keeps on, we should expect Hiroto truly falling for Sakura and the envious step sister revealing her the plot.
Intriguing.

OMG what did just happen in the end?

What is their relationship and why did he kiss her scarred back? Did he do that? I don't understand...

And at first I thought they where bro and sis....now what?