Okay its super obvious who Madame Ageha is right?

no it is not

at first i thought it might be the director's daughter (the sadistic bitch), but now i think it's her sister

I'm only on chapter 12, but I was assuming it was her sister. That is, she's not dead; she changed (caterpillar to butterfly). She blames her sister for all her suffering so wants her now to suffer.