Oh cmon, the book might be a misunderstanding, but having sex with Sensei after Futaba explicitly told her not to do that is definetly not.
She needs some consulting from Otoha
It seems like Nodoka, Otoha, Rin, and Sensei are still keeping that secret, but when that comes up, I
am curious what Nodoka is going to say. I can see Nodoka going for the loophole, that Futaba unintentionally gave her when she mentioned the fantasy smut novel condition, which Nodoka definitely played into:
The irony about Sisters, when it comes to Sensei, is that it seems that those he sees as Big Sisters (Sekai, Niki, and I guess Otoha), are those he's more willing to fuck. Little Sisters like Imani and Noriko (maybe Rin?) on the other hand, seem to be almost at the bottom of the fuckable category.
Nodoka may be some kind of sister/daughter/parrot/outlet thing, but she's an enigma even for Sensei.