So since there's not gonna be an update for a few more weeks, I started playing a red path to see if it had some interesting dialogues that linked to the story, and I found this in one of the beach trips, in the sauna (it's the event that replaces "no one can hear us").
Instead of Akira doing it with Ami, he refuses and Ami asks if it's because they're blood related. Akira says yes and then Ami replies "then I have bad news for you" or something like that. Then she says that if he bangs her, she'll tell him what she means. But of course Sensei refuses again.
This was earlier in the game, but we now know that Ami pretty much knows everything... so I'm thinking she knows at least another girl (that Sensei bangs even in the red path) is related to Sensei? Either that or she's trying to manipulate him...