Bella slips into the school by a side door; she just saw a thing and she thinks she's seen it in a book before and thinks it's supposed to be mostly harmless but before she wakes Giles up over probably nothing or kills a likely neutral demon she wants to check.
And she pushes into the library.
"I can double-check the prophecies I know about, but I don't recall any that sounded remotely applicable."
"I don't know her well. We've spoken a few times. I... have no idea what you should say, I'm sorry."
"You... seemed to like her, and, and trust her. I'm - I haven't been taking notes, you understand."
"Yes... I'm sorry," he says helplessly. "I've never had occasion to analyze your interactions from the perspective of 'how would Bella's sister react if she found out Bella no longer remembered her existence?'."
"Yeah, I'm just - in case I have been mindcontrolled, instead of it being everybody else, seems like it might be worth the extra sanity check."
And Giles continues futilely researching possible causes for an extraneous sibling or a comprehensive memory deletion.
"Sorta? Uh, Bella says you can't, like, analyze me or anything, but I wanted to make sure?"
"Being a - something? Like, you can tell if somebody's a vampire, can you tell if I'm a human in particular out of the kinds of non-vampire?"