So she pulls over and calls Giles.

"Yes?"

"Do you by any chance have an off-the-top-of-your-head diagnosis for my having a retroactively inserted sister, programmed into my phone, expecting me to pick her up from a friend's house, and turning up in other people's memories of my offhand remarks, who I can't remember having in the first place?"

"It's entirely possible that's what I need to hear right now but I swear to the Powers that Be In My Bedroom Sometimes that I don't remember her! Where would she go? Is she expecting to go to sleep in a mysteriously appearing second bed in my room or did Charlie's house morph to accommodate her or did my address change?"

"I believe you've mentioned said Powers waking her up..."

"Christ. Okay. What could've done this - either which way?"

"Deleting a sister from your mind: Powerful magic, I haven't heard of anything specific. Adding a sister to the universe: Much more powerful magic, I haven't heard of anything specific but it would be beyond the reach of most minor gods. There are classes of wish-granting demon that have effects this sweeping, but if this is a granted wish then I have no idea who could have wished for it... it does seem similar to a wished effect in some respects. A single central change with, with comprehensive secondary results. But in that case the only people who remembered the previous reality would normally be the person who made the wish and the demon who granted it, and I must assume you're neither."

"I don't remember wishing for a sister. I've never particularly wanted a sister. Do you remember me liking having a sister?"

"Yes. I've never had occasion to suspect that you were anything less than fond of her."

"...I don't know. But my instinct is that the latter is more likely."

"Is your instinct that because you remember meeting some girl who's, how old is she, who has sleepovers and shit, or because that is the actual better part of possible demonic shenanigans."

"I'm legit asking, you'd know and I wouldn't, but as the person here who does not remember some harmless how-old-is-she having sleepovers... reminding you to think about it."

"Patterns of demonic activity are what I was thinking of. I would expect... aesthetic differences, nothing I can easily pinpoint... if she were the source of the problem."

"Mm. So obvious options seem to divide into 'here's your ride home, Soph, I will be at the library all night, Slayer stuff' to postpone diagnosis; and 'Soph, you may or may not be demonic, please come to this not-Charlie's-house location for analysis'. Is there any sort of analysis to do?"

"I would be wildly out of my depth trying any kind of direct magical examination. The best we could hope for from that is the chance that she might, ah, reveal herself and attack, if we tried it."

"What could anybody possibly have to gain by giving me a sister or making me forget one?"

"I have no idea."

"...If you remembered her," says Miles, "you would value her highly, I'm assuming. Someone could have had someone or something they wanted protected at all costs by a powerful force."

"But I don't. Everybody remembers her but me. Phone, Watcher, ray gun vampire. - Giles, did you catch that?"

"Catch what?"

"Miles speculates that if I did remember her I'd value her and someone could have wanted something protected by, well, me. But I'm the gap in the memory revision, if she's a construct and I'm not just the sole victim."

"Yes... I'm not sure what to think."