Severitus fic idea where, as part of his reparations package, Sirius is given a special de-aging potion from the department of mysteries Time department where he's de-aged and the years that he was de-aged becomes murky and separate like he read the facts of those years and didn't experience it.
So, Sirius, newly mourning in an old world full of new people, is given to his cousin Andromeda for rehabilitation. He feels old and young and like he doesn't quite fit into his skin anymore but it only takes a few days for his resolve to follow Dumbledore's advice (and stay away from Harry until he's wrestled his grief under control) crumbles and he tracks Harry down, finds him huddled outside against the glass door in the Dursley's backyard, see him shivering in too big clothes that look like half of what he should be wearing on such a cold night.
But no one believes him. Andromeda frowns, saying none of her monitoring charms signaled his adventure. Lupin never replies to the hasty owl he sends. Dumbledore twinkles and assures him of Harry's safety. It's only when he sees a shockingly older Snape, remembers how the other man had begged forgiveness after the M-word incident, realizes he's not just a professor but a Head of House at Hogwarts and therefore must be considered trustworthy enough, that he asks - begs - Snape to check in on him. Sirius is old and young and knows he isn't able to do anything for Harry yet beyond asking for help.
It isn't until Sirius brings up what Lily would want - he's happy to engage in a bit of emotional blackmail if it means Harry's safe - that Snape's face twists into a disconcertingly familiar sneer but gives a short nod. Sirius hates how comforting it is: Snape's face is different but that look is an anchor in this new time and space where nothing feels quite real.
Sirius hasn't been cleared for regular life yet, isn't ready to be responsible for another human being, let alone Harry, can't imagine dealing with his new lordship on top of everything, so he digs up his best friend's will (and he can't even look at James' ugly penmanship without bursting into tears) and hands it to Andromeda to handle - thanks every star in and out of the Black family's preferred constellations his beautiful, kind, kind of scary cousin who's able to go be the adult he should be - and learns that Snape is the only other living name on the will.
Sirius grits his teeth, reminds himself about thirty times an hour that it'll only be for a month or two while he completes his rehabilitation, and signs the paperwork. At least Harry isn't with the Dursley's.
Andromeda convinces him - cajoles him then demands it actually- to visit with Harry every day. She says it's good for his health and - barring the way he chucked up three times the morning before the first meeting from nerves - he can't disagree. Snape's sitting room is packed with books and Sirius can't quite get comfortable with Snape behind him at the small desk as he tries to get to know this Harry.
Because this Harry is a stranger. This Harry is painfully polite and shy and doesn't quite know how to interact with him at first but when Sirius jumps up to get them some water and trips over the curling edge of Snape's rug he gasps then stifles a giggle over the ridiculous Superman pose Sirius had to throw out to save himself from a broken nose. Something in Sirius relaxes.
It'll be okay. Harry will be okay.
The days pass faster now. He sees himself filling out in the mirror. He feels like he fits inside his skin again. And he notices how Harry's health seems to mirror his. The child looks less fearful and more fearless. Snape is perfunctory but fair to both of them. It feels like stability and Sirius can't help but crave it sometimes. The three of them take little walks in the magically expanded backyard and Harry points out which plants he learned about, which ones are yummy, which ones "Sev'rus" told him to never touch. He begins to grab Sirius' hand, pull on Snape's trouser leg, ask for cuddles and snacks and his smile blossoms.
Sirius' heart hurts when he thinks about how James and Lily will never see this. He's still glad they never saw how he was on that first day when they took him back, though. Later, when Harry is washing his hands for lunch out back, Sirius hesitates then abruptly turns and offers his hand to Snape.
Snape, in that slightly infuriating way he has, slowly turns his head to observe Sirius' hand then redirects to stare at Sirius' face.
"I am aware," he intones like the judgemental berk he can be.
Sirius huffs. "Call me Sirius. I know this was supposed to be a short term placement until I'm released but it's clear Harry adores you."
Snape looks slightly shocked and discomforted by the thought. Sirius grins. He knows how Snape works now.
"He likes you. Merlin, I think I like you these days. You're clearly good for him. I know-" Sirius hesitates then tries again, "I know we don't have a great history but I'd like to try to get along. For Harry, I mean."
Snape's face is blank and Sirius doesn't know where or how to start. "Andromeda says I can start making plans. To bring Harry to live with me," he tries.
The outside of Snape's eyebrow has the most impressive micro twitch and Sirius feels like he's already stepped in it. "I, erm, I'm not ready, though. And I don't think Harry is either."
Snape's eyebrow smooths out again.
Do you," Sirius breaks off and tries again. "Would you consider a type of joint custody?"
Snape can't contain his shock this time, blinking rapidly, his eyebrows drawing together. "I beg your pardon?"
Sirius grins at his loss of composure, then wonders why he's so pleased about it. Merlin he's a Gryffindor. This subtlety stuff isn't for him "Co-parent with me. For Harry. I won't take you away from him. He trusts you. I trust you."
Snape's mouth falls open and Sirius is momentarily distracted by what looks like silver tooth. Is that some of the muggle densisty stuff that he learned about in Muggle Studies? Do they pour molten metal int-
"Severus, then." Snape cuts off that darting thought. "Call me Severus," and the hand that reaches out to Sirius is warm and real and Sirius knows he's touched Sna- Severus (pushed, maybe shoved) but it's new and a little bit exciting and entirely too fast to fully savor when Severus reclaims his hand.
And when Severus pulls out a piece of parchment and picks up a quill and slants that gaze at him, Sirius can't quash the thrill he feels or the smile that lets it into the world.