Ikr and not to mention something will happen in the future that kinda make their sibling status gone
Anddddd everybody also knows that they're not siblings, that's why the Lachia family doesn't accept ran as the head coz she's just basically an outsider whose mother was married to the previous head. And if everybody does acknowledge their status as siblings, in the era the story placed in ppl, specially the aristocracy, marries someone with a close- sometimes even closer- relationship with them so them being 'previous step-siblings' won't really bring much impact to the social world (also since the people that binds them together now is dead so the 'bind' has practically been dissolved) ╮(￣▽￣)╭
Yeah, and first of all, they didn't really treated each other as siblings especially Yustav.
The beginning of the story should've hinted you already where the story might lead you. By then, if you knew that this is not your cup of tea, you could've dropped it already. You don't have to reach this far just to say that "this is not the kind of story I want" or "Incest". This is NOT incest. They are only sisters/brothers on paper, not by blood. Yus already said from the beginning that he DOESN'T see Ran as family so DROP it if you don't like to see the progress. ╮(￣▽￣)╭