"It doesn't matter what you think of yourself now. It matters what Sumire thought. And she thought her sister was little miss perfect, isn't that right?"

That's the only way things make sense, but she doubts Midnight will be able to logic that out on her own. That would require her owning up to the fact that she is not, in fact, perfect little Kasumi Yoshizawa. Or after that seeing that Kasumi was not, in fact, perfect.