Hang on, hang on. My inner Bakemonogatari fanboy is still trying to formulate a rationalization of Araragi's stone cold sober grab-assing (grab-boobing?) of his underage sister. Oh! I got it. Maybe she had a mastectomy, and this was the only way to demonstrate that it had grown back. Ahhh... there we go. Cognitive dissonance averted!
The rest of the episode was awesome. Lots of Shinobu goodness, a great Kaiki reveal, and some good payoff to the new duo's sinister aura. Tsukihi's hysterical tendencies make her hilarious in a dark sort of way, and I was pleased to see the awl make a return. "Go away! I'm pretending I'm not here!"
It's interesting how the "genuine" duo of justice isn't so likable as the imposter Fire Sisters. They're lucky they're so powerful, or Araragi would have made hamburger out of them, vampire or not. You can hardly blame him for going feral on them; that was pretty cold, even if Tsukihi is a monster, as they say.