Sirius was injured in the battle at the Ministry, but not killed. This is set in the summer before Sixth Year.

Harry rubbed his eyes as he slipped quietly down the stairs of Grimmauld place. This was the third night in a row he had been woken in the early hours of the morning by his nightmares and it was starting to fray on his nerves a little. Pausing slightly as he neared the bottom step, he heard something from the direction of the kitchen and changed direction. The library would still be there in ten minutes, at least he could go say hello to whatever unlucky soul was also experiencing the silence that was three am. Harry snorted quietly as he appreciated his own joke. He really was tired.
Rounding the corner into the kitchen, Harry stopped dead. Frozen in the doorway, he could quite clearly see his godfather sat atop the dining table, leaning slightly forwards in a tight embrace and lips locked quite firmly only those of none other than Remus Lupin, who was clutching at Sirius' sides and stood on his tiptoes kissing back just as enthusiastically. Harry stared. When the hell did that happen? He moved quietly further into the room, trying to keep the blush off his face, pulling a cup from the cabinet closest the door. As he closed the cupboard, he heard a low groaning noise from the couple on the table. He whirled about, brandishing a teaspoon.
"Alright. I can deal with the kissing but the moaning stays upstairs. Got it?" he barked.
Remus span around, seeking out the intruder and blushing furiously when he saw Harry stood, mug and teaspoon still in hand. He raised a hand and pushed it through his hair as Harry noticed, for the first time, that neither he nor Sirius were fully dressed. Remus was stood in an oversized white shirt, mostly unbuttoned, and a pair of checked blue pyjama bottoms. Sirius was wearing a long knitted jumper which fell to just above his knees. Harry hoped that he was wearing boxers, too, but knew better than to ask. Some things should stay private.
"Um- Harry. Hi." Remus was still blushing, but seemed to have regained at least some of his composure. Sirius on the other hand was now leaning his head forward onto Remus' shoulder and shaking lightly. Harry couldn't see his face, but would have bet his firebolt that his godfather had fallen subject to a laughing fit. Bastard.
"Hi," Harry smirked, "having a fun evening, are we?"
"I thought everyone was asleep." Harry noted that Remus had avoided his question, but said nothing of it.
"Nightmares. Spent most my nights here in the library so far." At this, Sirius broke out into peels of laughter that could no longer be kept silent. Remus turned, if possible, even redder.
"The library?" he squeaked.
Harry simply sighed. He was far too tired for his godfather's antics, but he did feel a little sorry for Remus who looked absolutely mortified. He decided to take pity on him this once.
"I've only just woken up, was just heading down there for the night." He paused then, "I, uh- I take it nobody knows about this?" Remus paled a little and Sirius' laughter died at once.
"No."
Harry considered the pair for a moment. Sirius looked terrified, although of what Harry was not sure. He nodded.
"Okay, I'll keep quiet. But just so you know, I think Hermione may suspect something because she did ask me how close you two were, the other day. Of course, I was just confused, so-"
"Harry it's not that, I don't mind if Hermione knows. I've never told anyone." Sirius looked exhausted, Harry noticed.
"Nobody?"
"Nope. The only reason Remus knows is because I got drunk and threw myself at him in sixth year. I- I think I forced him back into the closet to be honest."
"Pads, you know I don't care." Remus spoke softly, taking Sirius' hand from where it lay on the table and gripping it tightly. "You come out when you're ready, alright?"
"I want to. I do."
"I know." Harry felt as though he was intruding as he watched his godfather's fear, Remus stroking his cheek lovingly.
"You kept this a secret from everybody for- for twenty years?" Harry was astonished. Here he thought he knew Sirius so well. Although, he supposed, he couldn't be angry at Sirius for being scared. Lord knows he knew what that felt like. Remus nodded.
"Your mum and dad knew, of course. Found out the hard way-" he blushed again. "But other than that, nobody knew a thing. It almost killed me when they died and Sirius got arrested on the same night- I had nobody left to talk to and nobody could understand everything that was going wrong so badly. I lost everything in one fail swoop. That night in the shack, I was honestly impressed with myself for not jumping on him- witnesses or not. I got him back. Pads wasn't ready to come out then, still a wanted criminal and all, but by that point I didn't even care that I couldn't tell the world so long as I had him again. Everything was okay again. Then last month-" Remus cut himself off as his voice hitched. Harry understood, he had felt the same way when Sirius almost fell through the veil- though of course for different reasons. Sirius wrapped his arms around Remus' middle and pulled him backwards towards the table, hooking his chin over one shoulder.
"Like I was gonna leave you again," he teased. "Somebody has to make sure you eat more than once a day."
Harry smiled as he watched his godparents. Godparents thought Harry, I like that.
"Well, I still won't say anything unless I am directly asked, how's that?"
"That's perfect." Sirius smiled at him, "Thank you."
"No worries," Harry walked towards the door, hot chocolate forgotten. "But, you best start planning."
"Planning?" Remus fixed a questioning glare on Harry, who smirked.
"Well, I can't let you have a better coming out than me, can I?" Without turning to see the astonished looks on their faces, Harry headed back upstairs to bed. He didn't get back to sleep, but he didn't stop grinning either.