Another piece inspired by asktheboywholived, who is very, very good at making awesomely inspirational stuff!
"So I get the official status of cool uncle, right?" Sirius said idly as he lounged around in the Potters' kitchen, trying to get in the way of Lily making dinner whilst simultaneously trying to look like he was trying not to get in the way of Lily making dinner. (Also known as "winding Lily up for sheer entertainment value".) James wasn't fooled, and given how pregnant Lily was right now it wasn't too smart to wind her up, but hey - Sirius could learn the hard way.
"I'm pretty sure that this kid gets to make up its own mind about who the cool uncle is," Lily replied, retrieving a spoon from Sirius's fidgeting fingers and giving them a light whack with it for good measure. "And I have my doubts about any child of mine being that entertained by you."
"Hey, I'm totally going to be the cool uncle!" Sirius protested. "And Moony's going to be the mooshy one who sits there making baby noises for hours, and Wormtail… actually he's probably going to either be terrified of breaking the poor kid, or turn out to be some sort of super child-tamer. …Is that a thing? A child-tamer?"
"It's a baby, Pads, not a bloody lion." James rolled his eyes. "I think the word you're looking for is possibly babysitter."
"I dunno, that sort of sounds like you're sitting on the baby, which I'm pretty sure is bad for its health."
"I can think of something worse for your health if you don't stop nicking the stuff I'm using." Lily snatched a pair of tongs from Sirius and glared at him in a manner that suggested they were about to be used to rip his nose off. He immediately tried to look very sorry.
"Okay, okay…" He darted a glance at James and added, "I still reckon I'll be the cool one, though."
"Where's my godson?" Sirius sing-songed as he bounced into the house, grinning broadly. It took all of ten seconds to track down Harry, who was in the living room flailing at the mobile dangling over his crib and gurgling in surprise at the noises that ensued.
James, looking thoroughly exhausted, was half-dozing in the nearest armchair, but looked up as his best friend entered and tried to look more alert. "Oh… hey, Pads…"
"Don't mind me, just came to keep your mini-you entertained. Go back to sleep." Sirius sat down in front of the crib, beaming, and ignored the incoherent mumble from James.
Harry stopped batting at his toy and fixed the universal bewildered baby stare on his godfather. Sirius beamed and held out a finger for Harry to grab. "How's my little man?"
"Noisy as hell," James supplied. "I think this is the first time he's stopped crying in days."
Sirius shrugged and allowed Harry to gum his finger. "How could anyone mind with a face this cute?"
"You forget about the cute face when he's been wailing in your ear non-stop." James reluctantly dragged himself upright in the chair to watch the proceedings, privately rather relieved that someone else was taking over baby-entertaining duty for a bit.
Sirius smiled blithely and allowed his finger to be chewed for a bit longer before retrieving it and moving the mobile out of the way.
"Hey…" He waggled his fingers as though doing magic the way Muggles sometimes did, then mock-gasped with surprise as he opened his hands to reveal… nothing.
For some reason Harry found this hilarious, and Sirius's grin nearly reached from ear to ear at the reaction he'd got. "That's my boy!" He tickled Harry under the chin, causing delighted giggles.
"So much for being the cool uncle," James muttered, amused at the performance.
"…He can't tell what's actually cool yet," Sirius said defensively. "I make him laugh, that's close enough."
"Mm-hm. Keep telling yourself that."
Sirius made a face at James and went back to coaxing giggles out of Harry. In his defence, it wasn't very hard.