But where? It's just assumption. And that "she is his sister" is just assumption too. Author is pretty bad with english, that's all. He suggests some things (like "do you want to fuck your sister\other dude?", but we don't know what he really means by that. Mystery. I know that she's strong witch (who can do transformations and stuff), and sorta have succubus abilities or demon in general. Fantastic creature, that was once a human.
Ok, say you're playing some sort of game. You have a friend with you, you are both humanpersons. You encounter a wolfperson. That's like a werewolf but instead of turning into a wolfperson only at full moon, it's just a wolfperson all the time. That wolfperson bites your friend (for whatever reason). Later, your friend turns into a wolfperson himself. And the first wolfperson tells you, yeah, she did that. Would you then assume, that that first wolfperson was always a wolfperson, or would you logically reason, that that first wolfperson was probably a humanperson at some point in the past and was turned into a wolfperson herself?
Alternatively: Say you're playing a different game. In the game, you have a friend with you, you are both alive. You encounter a zombie. I don't need to explain what a zombie is, there are people living in the amazonian bush who have never encountered a white person, yet who know what a zombie is. That zombie bites your friend (for whatever reason). Later, your friend turns into a zombie himself. And the first zombie tells you, yeah, she did that. Would you then assume, that that first zombie was always a zombie, or would you logically reason, that that first zombie was probably a living human at some point in the past and was turned into a zombie herself?
Do you want me to go on?