It isn't really spoilers; it said it in the first few chapters... but, she's not really innocent. She has a complex about being an illegitimate child and she [according to the novel this world is based on] will do really evil deeds and end up dying at the age of 20
That's why MC isn't kicking out her sister or killing her; she knows she's gonna die anyways so there's no point in intervening
tbh i really dont care about others at all. I just want to see our Holy Knight and Our Lady get together (๑•ㅂ•)و✧
Is her sister really is innocent? I have trauma when it comes to innocent looking girls HSINSICNS