Disclaimer - I don't own Harry Potter.
The original intent of the Upside Down Mirror series was to simply write a prequel for the original one-shot which was a story where Neville was the Boy-Who-Lived, which the other Upside Down Mirror piece definitely is, but it also turned unintentionally into a Regulus Lives project, but as I wrote the first chapter of the other one I got to thinking about Regulus asking for Sirius' help.
The reason for this comes from the fact I'd gone back to reread a few scenes in the books when I wanted to double check a few details and read the part where Harry describes Regulus as being smaller than Sirius, but then I got to thinking that Regulus might actually have been small enough, but at age eighteen he wouldn't have reached his peak in muscle weight either. I also got to thinking of how Sirius's disdain towards Regulus might not have anything to do with Regulus himself, but the fact he blames his parents and himself for what happened to his younger brother.
As for the autism headcanon, that occurred as I wrote the first chapter in that I realized the way I was writing Regulus, he did indeed have the traits of someone on the spectrum. It's not something which will be verbalized as this is a time when he wouldn't have been diagnosed, but to me it explains Regulus' behavior a lot.
Sirius opened his mouth to thank Lily for the lovely dinner when a sound like a cracking whip occurred and Kreacher landed in the middle of the table, upsetting various dishes almost as if purposefully trying to annoy Sirius yet create havoc for those his mistress looked down upon in disdain. "Bloody hell!"
Everyone at the table, which included not only the recently married couple but also their friend Remus Lupin stared at the House Elf standing in the middle of the dinner, not at all bothered by the fact he'd made a mess let alone splattered smatterings of food all over him. He also looked intently at Sirius which the young man did not at all like. Worse, he said, "Master Sirius."
"What the bloody hell do you want Kreacher? And you'd better not go revealing the location of my friends to anyone who might turn them over to Voldemort's side."
"That would go against Master Regulus' wishes."
Lily cleared her voice, nervous about speaking up given the fact the House Elf would not at all like what she said let alone acknowledge the fact she was speaking to him. "Is Regulus alright?"
Kreacher turned his head towards her, then said, "Master Regulus said Master Sirius would trust Kreacher's word that he was in need of Master Sirius' word if he acknowledged the Muggleborn and not do things such as calling her a Mudblood."
Which, in Sirius's mind still finding a way to use the word Mudblood was in fact Kreacher's way of getting around the fact he was being forced to associate with such a person let alone treating them with kindness. He wasn't quite sure Regulus was right, because even with that everything could just as easily be a trap where his younger brother would need to prove his loyalty to Voldemort, something he did not at all like.
"Did you not hear him, Sirius?" It was Lily who spoke while James continued glaring at the House Elf and Remus cleaned his mouth looking rather amused at the situation. "Your brother needs your help, and if you're actually thinking for one moment that Regulus would ever turn you over, then you are a stubborn pig."
"Kreacher thanks the Muggleborn for the compliment paid to Master Regulus."
"You're just being conned by this…"
"You don't know your brother, but do you really think anyone on the other side would think of using a House Elf in this manner?"
Sirius opened his mouth to say that he felt Regulus would likely think of such a thing, but then quickly remembered Regulus always shared a rather odd bond with the creature standing on the table in front of them. He'd never understood his brother's fondness for the creepy creature who would spout his mistresses rhetoric with unabandoned glee, but then again, Regulus was always a bit odd.
And, he loved his brother, which honestly made him hate his parents, even more, seeing how they influenced him and were to blame for Regulus becoming a Death Eater. He'd never felt this fit Regulus, but the way his brother collected clippings regarding Voldemort with such childish zeal; when he'd tried calling his younger brother out on this, Regulus added a sign above the door saying "Do Not Without the Express Permission of Regulus Arcturus Black."
Of course, that sign wasn't just added because he would enter his brother's room to see what he was doing. There were times when Sirius would step into Regulus' room simply because he wished to spend some time with his brother despite their differences that their mother would step in and start yelling at him for pushing his ideas onto his younger brother, let alone what Sirius said regarding the clippings.
For Regulus, their mother's yelling was far worse than anything Sirius could say or do, but his one and only regret about leaving the home at the age of sixteen was leaving Regulus behind under the influence of his parents.
One of Remus' eyebrows shot up, still finding the situation amusing. "That was easier than I expected."
"Despite his feelings toward everyone else in the Black family, Sirius does care about his younger brother," James spoke up, making Sirius remember how he'd cried himself to sleep some nights telling James just how worried he was about Regulus being stuck in that home, something he didn't want others to know, though somehow he knew Lily knew even without James having told her. Lily simply was that way.
In fact, if it weren't for Regulus taking on their parent's pureblood concepts, Sirius had the feeling his younger brother would have followed Lily around Hogwarts like a little lost puppy and taken her on as an older sister figure. A sigh came about from more thoughts of how Regulus, who was a very sweet child growing up really wasn't suited for the life of a Death Eater. He opened his mouth to say something when Kreacher beat him to the punch.
"Mistress was very heartbroken when…"
"Okay! Okay! I'll go. Please don't talk to me about my mother Kreacher." Those words he felt were leading to a very awkward conversation between him and the – well, the creature. He looked at his friends with a slightly pleading look wishing that they would in fact go with him, making some of his anxieties about going with the House Elf go away.
Because even though Lily insisted it might not be a trap, it might still be. The worse thought though was that something bad happened to his younger brother, but in the back of his mind, he swore he'd never forgive his mother. There were definitely a few things he needed to ask of the now fidgeting House Elf. "Kreacher, I order you to tell me the truth."
"Yes, Master Sirius. Master Regulus already ordered Kreacher to do so."
"Is this a trap?"
"Is Regulus hurt?"
"But he does need my help?"
"Master Sirius is the only person Master Regulus can trust with this, he told Kreacher to say."
Which only made Sirius and likely his friends he would soon be leaving behind wonder what kind of trouble Regulus managed to get himself into. "Okay. Then, we'll leave for…"
He'd not expected the House Elf to Disapparate him away from the place.