It is a big difference, but if anything she probably likes Molly more. Plus, if you replace her with Karin, it makes it more taboo, which probably makes it hotter in Kirin's eyes, since she wants to fuck her sister, and is into incest. Kirin also cared about Miku and Noriko, and probably Ayane, but that doesn't stop her from trying to use them to get herself off.
Kirin was worried because that's probably the first time she's ever seen Karin like that, but we didn't see how she would react if she actually knew what happened.
Also, her getting off on actual sexual assault, blackmail, and rape makes it hard to see how you think she's not a monster.