This Is My Truth, Tell Me Yours
There's only one story in the entire world and everything else is a variation thereof, so the idea that Sirius fell through the veil and died is just one potential ending out of hundreds of thousands.
Remus doesn't like that particular ending, so he's going to choose another one.
Lots of others.
#1:
When Sirius fell through the veil, he literally fell through it and down the stairs behind it. He was knocked unconscious during his fall, which he's still incredibly bitter about, and at night he keeps Remus awake with his lamenting and whinging about not getting a chance to 'really put Bella's knickers in a twist.'
He'll keep at it all night if Remus doesn't shut up, so Remus rolls over and pushes and climbs until his body is covering Sirius'. Then he kisses him again and again and again until he Sirius forgets what he was complaining about in the first place. If Sirius is really riled up, Remus will slip his hand into his pyjama bottoms and stroke Sirius' cock until his eyes roll back in his head and he starts whimpering. After Sirius comes, Remus will kiss him on the forehead, wipe his hand on the bedclothes and go to sleep.
#8:
It's always unnerving when dishonest people tell the truth. Although perhaps 'dishonest' isn't the right word for Sirius: cagey, morally-vague, reticent, evasive are much more tactful, but Sirius has never been very tactful. In fact, when he confesses, accuses -- when he says that he thought for a brief moment of time that Remus might've been the spy, all Remus can do is stare.
He knows he should be railing and shouting and doing any number of things, but in the face of such blatant honesty it's hard for him to do anything but marvel.
They have never been good with confessions; they have always had to stumble along until the truth has decided to give them both what for, and the fact that Sirius is being honest raises enough hackles that Remus wonders if he's under Imperio or if there might be a Polyjuice at work.
His wand is out before he can think better of it, and when Sirius starts laughing, Remus knows this is no imposter. Only Sirius would laugh in the face of death. Sirius is the only one who's been laughing since the war started.
"I thought it was you," Remus says quietly before slipping his wand back into his robes.
"I know." Sirius sobers up quickly enough. "But we can fight about that later."
#21:
There are muggle prophylactics gathering dust on the end table because Sirius wanted to try them out, but Remus thought they were entirely too much work and after the first time Sirius slipped one on and realized that it felt a bit like Lily's Wellies, he went right off them.
But they sit there, day after day, week after week, year after year collecting dust until Remus and Sirius have five-year old Harry over for a visit and he wants to know why Uncle Remus keeps his balloons in little wrappers. After that they go in the rubbish, but the following morning Sirius wakes up to find the bed filled with condoms in all colours and sizes. He's not amused, but Remus thinks it's a great joke. After charming them all away, except for the glow-in-the-dark one because Sirius has to try that one out, they take Harry to the zoo.
The beautiful thing about Harry, Sirius likes to point out, is that he can be returned at the end of the day.
#6:
James's nose is nearly in his tea and he looks up through his fringe wearily. "If you're sure," he says again. "I just want to make sure that you're both sure. This is a serious accusation. I mean this this is Peter you're talking about here. Peter Pettigrew. Wormtail. He's a Marauder, for Merlin's sake."
Remus and Sirius look at each other over his head and nod. "We're sure," Remus says as the stairs creak behind him to announce Lily's arrival.
"Sure about what?" Lily asks, shifting Harry from one hip to the other and pushing a wisp of red hair behind her right ear.
#37:
Remus doesn't see Sirius fall, but he feels it. He feels the invisible cord between them pull and tug and go someplace he can't follow, and when he turns towards the veil and sees the fabric billowing he knows. He just knows.
So he follows.
#2:
Part of love is knowing when to just walk away before something gets thrown at your head, or your chest or your knees, but apparently no one ever told that to Sirius. Apparently no one told him that it was a bad idea to fall in love with a werewolf, either, which is why he is always available when the moon is full. For Sirius there are never other places to be or appointments that are more important. There are never girlfriends to placate (James) or exams that must be revised for because someone has to pass Runes (Peter). Sirius is always available when Remus needs him. He is never far away. There is never anyone else more important to him than Remus.
Never.
It's always Moony and Padfoot. Always.