Sirius is quick on his little brothers tail, sprinting as fast as his two human legs allow for. So he sees the way his brothers steps get smaller and uncertain, wobbly and lacking the self assurance the Black family taught them both.
His brother hasn't regressed in two weeks, which is a long time already for Regulus and had Sirius immensely worried.
But he probably has been fighting his headspace the whole day and to walk in while Sirius says THAT about Snape....
He messed up big time, he is going to verbally beat his own ass so bad, but priority is getting to Regulus and trying to fix this.
"Regulus!" He yells, but his brother, quick as a mouse still, evades his outstretched hand.
He whimpers as if burned and takes a sharp right that makes Sirius accidently widen the gap between them. His ankles aren't built like that, alright? He sprints in an empty classroom he just saw his brother disappear in.
There he is, hugging himself in a self soothing manner he has seen their mother do a hundred of times before, and steps closer.
"Don't hit me," Regulus whimpers, softly carving himself into a ball and Sirius gaps dumbly.
"Reg I would never!"
"But you talked about it! I don't want to get the belt." He whimpers, crawling away from Sirius. That's when his legs give out, entering his newborn headspace of between six months and two years, trying to crawl away.
Sirius is on him in a second, drawing him into his lap and realizing he wet himself.
"Reg, I would never hurt you." He whispers shakily, kissing his brother on the forehead and taking off his student jacket. Every classroom has a supply cabinet, so that's where he finds the things he needs to clean his brother up, which he does in under two minutes.
He has done this for Regulus ever since he was five years old, being taught how to by his mother. He remembers jumping on his tippy toes, begging and hackling with her to let her help him take care of Regulus. He was so fascinated with the little thing he could call his own little playmate, as their father called Regulus jokingly, that he wanted to be his whole world.
His brother, his responsibility, his purpose, his Little.
His, his, his? His!
He would kill before he lets someone have his brother, his baby.
Sirius has always been proud to follow his father's footsteps in becoming a primary caregiver and fulfilling what his father couldn't.
His father is one of the biggest scandals in the Black family and was forced to suppress his natural instincts violently enough that there is barely any instincts left in him.
It took a huge tool on him, his hair already grey and matted, wrinkles on top of wrinkles. The glassy shift of his eyes used to scare Regulus when the boy was barely able to walk and he used to cry for Sirius whenever their father was around.
Regulus becoming a Little was ... not entirely surprising, but their father just can't know. If he did, he'd collapse on the onslaught of pheromones he'd release in an attempt to reconnect with his inner caregiver.
Their mother doesn't really care. She'd be disappointed no matter what the two of them would end up as. Sirius isn't even sure if he knows what his mother's designation is.
But a Black caregiver is a possessive thing, he can't stand other caregivers just looking in Regulus' direction. He needs him to be his and his forever, Regulus can't leave him. He won't let him.
He has his little baby redressed in a minute, having transfigured his clothes in something more comfortable. Everything is green colored. (Sirius made the mistake to cloth his brother as a lion only once; his ears are still ringing from that particular incident, so never again, even if his brother looked absolutely adorable.)
Reg blinks up at him, thumb securely in his mouth, and Sirius coos down at him.
Then, Regulus starts frowning, looking shockingly eerie like their mother.
"Dun hit me." He mumbles between his thumb and lips and Sirius scoffs.
"I let you continue pretending to be a mean Lil blood supremasist, do I not?" And Reg blinks up at him.
"Look, I was talking about Snape and that's different. I don't care for that gi-" he hesitates, looks into the ridiculously young eyes, and corrects himself lamely, "know-it-all. I love you though and would never do that to you."
Just then, Remus walks in and Regulus, whose regression is kept secret from every and anything, except Remus, holds out his arms for the werewolf.
Sirius almost growls at Remus, but holds himself back. He thinks.
Judging by Remus' eyeroll, he isn't all that convinced he managed.
"I brought the invisibility cloak." He croaks, holding Reg near his hip. Sirius loves Remus, but his brother and him take after their super tall father, so Regs toes still reach the floor.
"Here, gimme the baby-"
"Not a baby!" Said baby protested, but let himself get transferred to Sirius' arms. Something inside him unwinds heavenly and he takes the cloak and draps it over himself and Reg, trailing after Remus who is securely walking towards their dorms.
James won't return today, Sirius thinks as an afterthought. He briefly thinks of him and that weird situation with Snape, but disregards it in favour's of pretending like he isn't hiding his baby behind a cloak, showing off the prettiest and cutest boy to ever walk the earth.