What previously happened
Sirius Black was three when his mother Walburga Black first used an unforgivable curse on him. She'd demanded of him to stop misbehaving, he didn't know what he was doing wrong. His mother used the cruciatus curse. Walburga later said that Sirius wasn't sitting upright.
His brother Regulus Black was forced to watch, he was only one year old.
Walburga didn't stop and continued with this punishment if they "misbehaved", if they made mistakes, if they didn't write legibly enough or if they didn't pay enough attention during learning time. The Black brothers didn't even have a break from "home" at school, because they had private tutors or were taught by their parents.
By the time Sirius was 10, he was fluent in french.
When Sirius got into Hogwarts school of witchcraft and wizardry he was scared but happy. He didn't want to disappoint his parents, yet he also didn't want to be placed into the Slytherin house.
At Hogwarts Sirius met the people he considered his family: James Potter, Remus Lupin and Peter Pettigrew. He found more friends over the years but those were his closest friends.
Around fourth year Sirius started becoming suspicious about Remus Lupin's monthly disappearances. He started reading about child abuse and human trafficking. He was worried sick.
When in defence against the dark arts they learned about unforgivable curses and how no one is permitted to use them under any circumstances, Sirius finally had to accept that it was him who was being abused. He cried for the entire next day.
Sirius confronted Remus the following weeks about everything and presented his theory of child abuse. He begged Remus to tell them what was wrong.
Remus couldn't keep it a secret anymore and told them that he's a werewolf. They all just accepted that, no disgust and no hate, just trust.