See, there's multiple "messes" and you don't even realise it.
This one has nothing to do with people asking a question, and everything to do with something you said.
What you call "step cest" in this game,
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aside from being a nonsense joke that's a nod to "not incest" porn, is not the case in this game at all, because you don't actually understand what step family is, despite it being explained to you.
Marcus and Emily weren't married, so Mira and the MC are never step-siblings regardless of whether or not the MC is Marcus' son.
Even in the case of step family where the parents aren't married, Marcus and the MC did not live with Emily and her children in the same household.
When the player chooses to have Mira and the MC both being Marcus' children, they are half-siblings, which means they are biologically related, therefore incest.
Now, F95 adopting a similar stance to the laws of some countries does not make their definition correct.
In fact it only serves to regard the tag system as a guideline and not "letter of the law".