"If we die and leave Harry in a book he stays there forever. If I kill you you just die a normal amount. I really really really do not want to but if I have to I will." 

Fuck he is not kidding even a little. 

Remus can take Sirius in a fair fight. It's not even that hard, comparatively; he's had the same blind spot since he was fourteen and learned a cool hex-sequencing trick that works on everyone who didn't watch him learn it.

Remus absolutely cannot take Sirius and Voldemort. 

"Fine." 

So he holds hands with the horrible cockroach and his prisoner, and waits for Remus to weave the Vow, and then he waits expectantly. 

She has a roughly hour-long detailed specification, memorized in contract format. The short version is that she hasn't lied to him (today (but is not committing to not doing so in the future)), really does have Harry Potter (the one with which he is familiar (still alive)) soul-trapped, and is in fact agreeing to release him (uninjured (et cetera) in exchange for the knowledge of the Fidelius Charm (defined as the spell with the following function...)(*). 

She recites it in its entirety without pause or consultation of notes, because why not show off, if you can. 

(*) Historical note: the only people currently able to teach the Fidelius Charm - a strict subset of the people who can cast it, because the knowledge is itself charmed - are Albus Dumbledore, Sirius Black, Andromeda Tonks, and the Baba Yaga (**). In most timelines it would have been possible to bribe this knowledge out of Arcturus IV as recently as last year (as Dumbledore in fact did many years prior), but Harry assassinated him before Voldemione could get to him. The people he personally taught the spell to thence became Secret-Keepers, which would have included a few more of his less-good-aligned grandchildren and grand-niblings if not for Andromeda having gotten away with Obliviating her sisters when she was fifteen.

(**) Additional historical note: This happened in the 50s. It's probably fine.  

He was honestly kind of hoping she was lying and they could instead fall back on the plan where he and Remus try to kill her, which would probably not work but at least it would feel less like shredding all that remains of his principles.

He agrees to the vow. 

It doesn't even hurt. It kind of seems like it should. 

It takes, on average - and this is an average of a distinctly non-average population, mind - six months to learn to cast the Fidelius Charm properly. This has very little to do with the quality of the teacher and quite a lot to do with being able to fit enough math into your brain as a single chunk, because if you try to pick up any of its component parts separately, the spell detects that you don't already know it, counts you as someone who is not supposed to know, and every piece individually forgets itself. 

Voldemort would have taken about six weeks to get it down, doing very little else. 

Hermione Granger gets it in four. 

Unfortunately (?), it turns out what happens if you both do and don't count as a legal target for a Fidelius Charm at the same time is quite a large explosion. 

Welcome to the void between space! There are no things! Please enjoy your infinite and also instantaneous stay. 

"Was that you?" 

"No. Yes? Maybe???" 

Ah, they are going back in time. Why is this a thing that keeps happening. 

"I am going to murder this horrible child's entire family so that she is never born." 

Probably she's going to stop being able to talk again when they reenter normal reality but that is future her's problem and present her has shared a brain with Tom Riddle for too long not to be tempted to gloat a little. "Not if you want me to teach you the spell you're not! Technically Sirius just finished fulfilling his part of the deal, you didn't specify which inhabitant of this body counts while you were neurotically making sure he couldn't cheat by teaching it to Nagini." 

Which she successfully avoided noticing until it was too late by focusing 110% of her attention on the shiny new spell, you see. 

Oh, he sees now why Harry was so upset about this girl probably dying, she's a delight. 

"Also he probably can't kill Lily and James until they have Harry since that is the only way to fulfill the vow to present Sirius with an alive Harry," she adds, thoughtfully, to Sirius. "You know. In case that's relevant to your strategic planning in a minute." 

"Incredibly relevant, thanks." 

Uuuuugghhhhh. "Someday I am going to Cruciate the lot of you for thirty consecutive years and you will regret all of your choices." 

"Believe it when I see it and not before, motherfucker, you don't scare me. ... And Granger, you are an absolute hero, I am so sorry about the horrible war he is about to wage with your hands, we will get you out of there some day, all right?" 

The funny thing is, yesterday she didn't think they'd ever manage it. Today she kind of thinks they might. "Thanks, Sirius. See you on the other side."