"How can you not remember hitting someone?"

Does that happen? I'm curious. I suppose if you were caught up in a moment of chaos or distraction, it might be possible to forget striking another person, sure. But if you're a sister who was just playing with her brother a moment earlier, probably not. And that same sister running into your room under the guise of just wanting to say hi makes it even less believable. And the little brother following closely behind angrily calling out his sister's name adds to the dubiousness of this claim. So...after very little contemplation I've come to the conclusion that no, her claim of momentary amnesia is not valid and has now, as a matter of fact, added another element to her punishment. Case closed.