Hibiki, I'm pretty sure they were like that before finding out that Akatsuki got the first kiss. All that knowledge did was push it into overdrive. You don't wake up funny in the head.

I think akatsuki is saying そんなことじゃないよ

Fubuma said:

I think akatsuki is saying そんなことじゃないよ

One of the Pixiv commenters thinks it's な、なんのはなしよ, "Wh-what are you talking about?"

Her condescension to her sisters hitting puberty is delicious.

Moonspeaker said:

One of the Pixiv commenters thinks it's な、なんのはなしよ, "Wh-what are you talking about?"

That makes more sense.