Karin's definitely adverse to lewdness, and couldn't even stomach seeing a girl touch a boy non-sexually:
Karin also seems to have androphobia on top of her innocence and purity.
Frankly, unless she trades her body and morals for her sister, I don't really see how else she'd ever "willingly" be with Sensei, sexually. Especially after what happened. Albeit, if Sensei reinvents himself or gets reinvented, then I could see something happening.
The generics seem a little out of order. Ami's, for example, couldn't happen until at least next update, but they've been prepared for a few updates now. Karin's may have been intended to take place before this one, but wasn't implemented yet.
Although, perhaps Karin will simply be reset, like Maya, and Sensei gets to have a better chance at grooming her? Kirin would probably be the only one besides Sensei that would notice.