Sirius could face serious trouble. Snape would surely report him to Dumbledore. Dumbledore might have to take action like restricting Sirius' access to Hogwarts or even sending him back to Azkaban if the harm to Harry was severe enough.
Snape might be secretly pleased at first, seeing as he has no love for Sirius. But then, being a teacher, he'd probably feel obliged to report it. He also might use this situation to his advantage against Sirius in the wizarding politics within Hogwarts.
Socially, it could be difficult for him. If others find out that he still has feelings for Lily and she's pregnant, he could face ridicule or be the subject of gossip. Also, depending on his relationship with Lily's partner, there could be conflict. He may have to face the fact that he has to keep his distance more than ever to avoid causing trouble for Lily and her unborn child.
Well, in the fanfic, Harry might find himself in a very different power dynamic. With Tom Riddle knowing about his abuse, Harry could be pushed into a corner. He may be forced to rely on Tom Riddle for protection, which means he has to follow Tom Riddle's rules. This could lead to him losing his own sense of self and freedom. Also, the knowledge among others that Tom Riddle is involved with Harry due to the abuse situation might make Harry a target for those who oppose Tom Riddle, putting his life in even more danger.
You can try some popular fanfiction websites like Archive of Our Own (AO3). Just use the search function on the site and type in 'sirius finds hurts harry'. There's a good chance you'll find relevant fanfictions there as it has a vast collection of various fan works.
In the fanfic, Snape's reaction could be multi - faceted. He might start by investigating the abuse more deeply. He could use his skills as a Potions master and a former Death Eater to find out who is behind the abuse. And then, he might take steps to ensure Harry's safety. This could involve enlisting the help of other teachers or even trying to get Dumbledore more involved. He may also start to teach Harry some extra skills or give him potions to help him deal with the emotional and physical trauma of the abuse.
Sirius might use magic to punish them. He could hex them or use some other form of magic to make them feel the pain they inflicted on Harry.
First, be very specific with your search terms. Use the exact phrase'sirius finds hurts harry' in the search bar of fanfiction platforms. Also, check if the platform has filters like genre, rating, etc. and use them to narrow down your search. For example, if you only want to read completed works, set the appropriate filter.
They might face magical punishment. For example, they could have their magic restricted or be sent to Azkaban if it's a really bad case. McGonagall would not tolerate such behavior and would push for the strictest punishment.
Maybe Snape uses this discovery as an opportunity to teach Hermione something new. He could be curious about how she got the condition on her arm and offer to show her a spell or potion to heal or manage it. This would also be a chance for him to show a different side of himself, not just the strict professor.
Maybe Sirius left his possessions to Harry. His beloved motorcycle could be bequeathed to Harry, as it was something very special to him and also had a connection to Harry's escape from the Dursleys in the past.