"...G-glowy green," says Soph slowly. "And a - a pretty bird, I didn't know what kind it was."

"...And?"

"Cowbirds are like cuckoos. They lay their eggs in other birds' nests and expect the other birds to look after them like they belong there."

"Or perhaps," says Sherlock, "to put it another way, they are laid as eggs in the nests of foreign species and are dependent on the host family for sustenance and protection until they grow to adulthood. Two hours ago you did not have a sister, dear Juliet, but now you do."

"By what logic? Maybe if the spell had worked on me and we found out some other way I'd call it close enough for government work, but she's a stranger to me."

Soph sits down on Canada, turned away, sniffling.

"What am I supposed to do with her? I have not had - how old are you supposed to be?"

"...Seventeen."

"I do not have seventeen years of experience of being her sister."

"Develop a counterfactual pastwatch. Study up. I'll spot you the hex."

"Those monks who were casting this spell wanted me to protect something. This was exactly what they were hoping for, except they didn't expect me need to do homework on the subject. Shouldn't we find out what she is first?"

"I'm not anything! I'm not - anything, I'm just - I'm Soph," says Soph. "Lie-detect me, I'm Soph, I'm Soph, I'm not going to hurt anybody!"

"You're not going to try to hurt anybody. What if you're just - rigged to explode? We don't know."

"Fine, then," says Sherlock. "I agree that finding out where she came from is the more urgent priority. Let's do that. In the meantime, however, I commend your paranoia but suggest you temper it with empathy."

"Do you have any practical, object-level suggestion on how to do that?"

Soph shifts uncomfortably. "I dunno, you could - pretend I'm somebell else's little sister? And you have to look after me for - for her, because I'm stuck here. You want to look after me for the one who had a - me. And you should - expect me to know about that many things about how you are and that I grew up in a Sunshine family world and that sort of thing but - I dunno. That's the only idea I have."

"It's effectively true," says Sherlock. "With one small alteration: Juliet is the one who is, for most purposes, stuck in the world of another Bell who is like her in every way except for having a Soph. This means that, for example, if Soph is rigged to explode and does so, you can expect a funeral at which your parents will cry."

"Well," says Bella, looking up at the sky, "...that would be bad. I mean, I can sometimes door Downside, but not always - she looks torchable but if I torchable her I can't undo it if it turns out to be a bad idea -"

"I've survived this l- I mean, um, if I were on a tight schedule I'd have exploded already, right? I can wait while you figure me out."

"It also seems unlikely that the mystery monks would have gone to such trouble to protect her just so she could turn around and blow up," says Sherlock. "Not impossible, but unlikely. Shall we do some more spying, then?"

"Yeah." She turns her pastwatching on Bath Lady And/Or Shower Dude as of when the former slaughtered the monks, and follows from there.

Bath Lady killed all the monks, then threw an enormous tantrum in front of her cowering minions about how the monks had managed to conceal the Key before she got there and now she'll have to find it and she doesn't know where on Earth they could possibly have hid the thing. Around the time that Juliet was discovering her new sister, Bath Lady was just winding down her tirade.

"So, Soph is something called the Key. Doesn't sound explodey, I guess." She watches.