Another reason could be that Sirius is trying to protect Harry in a strange way. He might think that by disowning him, he can keep Harry away from some greater danger that is related to Sirius himself. Maybe there's a powerful enemy after Sirius, and he believes that if he cuts ties with Harry, the enemy will leave Harry alone.
A common reason could be misinformation. In the wizarding world, rumors can spread quickly. If a false rumor about Harry being involved in dark magic activities got to Sirius, he might disown him. For example, if someone planted evidence that Harry was secretly communicating with Death Eaters.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
One reason could be the strong bond between them in the original story. Fanfic writers might think that making them father - son would enhance that bond further. Also, Sirius was a very important figure in Harry's life, so it makes for an interesting twist.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
One possible reason could be a misunderstanding. Maybe he discovers some secrets about his parents that he misinterprets. For example, if he finds out they were involved in some dangerous or morally ambiguous activities in their youth that he doesn't fully understand at first.
One possible reason could be that he feels unsafe at Hogwarts. Maybe there are threats from dark wizards within the school that the teachers are unable to fully protect him from. Sirius, being his godfather, represents a safe haven.
One great 'Harry Potter Sirius Black fanfiction' is 'Padfoot Returns'. It really delves into Sirius's character and his relationships within the wizarding world. The story has a great plot that keeps you hooked from start to finish.
There probably is. The Harry Potter fandom is huge and full of diverse ideas. Writers might explore this idea to add new twists to the story, like how Harry's relationship with Sirius would be re - imagined if he were his daughter, and how it would change the overall plot dynamics.
In some fanfiction, yes. Fanfiction writers often like to play with the relationships in the Harry Potter universe. They might create a scenario where Sirius Black is Harry Potter's father, perhaps exploring how this would change the story's dynamics, the relationships between characters, and the overall plot.