And she actually DID that to her! The finger-fun "down there" was of course not shown on screen, but she was shown to move her finger towards Satsukis croch, the camera went up and Sastuki was shown to make an aroused expression.
...was this all really such a big shock that people have to bring it up on every picture like when people noticed that Kagerou looks like Holo when she holds an apple? I haven't seen a second of this show that wasn't an animated GIF here and I could tell it was going to get ripe. Obviously didn't have enough info to predict the incest business, but I'm still not that surprised...