I am calling it, she skinned her real papa and will do it again.
Why? Let me explain.
In the current timeline, we can undoubtedly conclude two things about her, one she is obsessed with her current papa, and two she is batshit crazy or psychotic. In the previous chapter, we have already seen her peeping on her parents having sex and she seemed "turned on", but despite doing such a bizarre feat, she is yet to be seen doing any sexually suggestive behavior in front of her papa or off-screen. So I don't think she is actually sexually attracted to her papa, if she were, she would probably do cringe anime girl shit but strangely enough, she did not. Instead, she seemed very possessive of her papa saying things like "Papa is mine", "Don't you like me?" or "We are a family" kind of things, which is really strange coming from a stepchild which goes without saying. So you might be asking, why do I think she would skin somebody? Let's go back to her parents having sex, it is the only time she can actually see her "papa" 's bare skin. Not only that, she also sniffed his body and in the first chapter, she took a liking to him at a first glance, before even knowing him properly. Not to mention the "sharing our pain as a family" part. We mostly feel pain through our skin, and she speaks about sharing that pain. And now in the current, she has a "secret", that secret is what she does in the summer vacation, Where does she vanish? What does a girl like her have to hide from her friends and possibly family? It is quite clear now that she lacks empathy considering how she dealt with that jealous girl. So My theory is that she skinned her real papa and stuffed it somewhere hidden, and she goes to visit her "papa" or even wears her papa's skin in the silence of the lamb's style, and this time she is thinking about broadening her collection.
Yeah, this theory is horse shit, but I still think there is a high chance that she is a serial killer.