She would probably more likely hang out with molly as with her sister. And sure, i wouldnt be surprised if she had fantasies about that sort of things with her sister. But having fantasies and actually seeing her crumble because of it is something different. And with miku it is something different, that feels much more like something she wants to share. I actually believe in her fucked up mind, she was doing miku a favor. And with ayane there was revenge that played into it. And neither she nor sensei did rape her. She just forced her way into it. Sure that is fucked up, and i never did deny that this is what she is. But i am pretty sure she did justify it in her head in a way that it wasnt as bad as it looks from the outside.
The fact alone that she was worried shows one thing pretty clear. And that is that it doesnt makes her happy to see her sister suffer. We dont need to see what she actually said, or did right after. Because no matter what, it would have been a follow up to her been worried about her sister.
She is like all of the other girls broken. In her own mind probably even beyond repair. And when then a situation comes that is a one in a million chance like ayane, she is going to take it. And then is just no longer able to let go. But in the end, she never actually did anything proactivly to really hurt someone. I mean if she really wanted karin to suffer, she would have pushed sensei into that direction. She would have planted a seed in his mind to rape her sister, to really mess her up. But, she didnt do this. The only fucked up sex she did actively pursuit, is and was her own. And that is why i dont think of her as a monster. Real monster dont just wait for chances, they create those chances.