Responses

That's not the real cossette,.a devil is possessing her body. The real cossette is already dead and it was all tha fault of the father. You can't stop them to hate her because I myself hate her because she's a devil. She doesn't have any backstory, the real cossette has a backstory but not that devil who is causing chaos right now.
Frankly I see a lot of people hating on her already but for me it depends on her backstory. It is already established that she is actually related to Keira, unless this is another Jeanette scenario form who mad me a princess (Cosette really reminds me of Ana, from a character design point).
So if she had a really shitty life while her 'sister' lived in luxury albeit not loved, there would be no reason to hesitate in killing her to take that place. Not that it's right, but unlike the father it has no betrayal or unnecessary malice underlying it