There are several potential themes. Forgiveness could be a theme as Sirius may have to forgive himself for any wrongs he believes he did to Regulus. Another theme is the power of love within a family. Even though they had their differences, there was still love between them. The theme of mortality is also relevant as Sirius is faced with the finality of his brother's death. This can make him question his own mortality and the purpose of life. Also, the theme of discovery is possible as he tries to find out more about what happened to Regulus, which could lead to new revelations about their family or the wizarding world in general.
Sirius might be completely devastated. He could go into a state of shock at first, not believing the news. Then, he might be filled with guilt for not being there for his brother, and anger towards the situation that led to Regulus' death.
When Naruto discovers Jiraiya's death in fanfiction, it's a moment of pure shock. He may immediately recall all the lessons Jiraiya taught him, from the Rasengan to the importance of never giving up. This news could fuel his determination to become stronger, not just for himself but also to avenge Jiraiya. He might also have flashbacks of their adventures together, which only intensifies his pain.
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.
Well, James is a loyal friend. So, when he discovers Sirius is abused, he would most probably drop everything else he is doing to focus on Sirius. He'd be angry, not just at the abusers but also at himself for not realizing it sooner. He'd make sure Sirius knew that he wasn't alone and that he had someone who would always be there for him. James might also encourage Sirius to learn self - defense magic more intensively so that he could protect himself in case the abusers tried to harm him again.
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 common theme is family loyalty and betrayal. Since they come from the Black family, their actions towards each other and the family are often explored. For example, Sirius leaving the family while Regulus had a different path within it.
The themes often revolve around the idea of secrets. Regulus may be hiding secrets from Sirius, which ultimately lead to his departure. This can create a sense of mystery and suspense in the fanfiction. Another theme is the power of choice. Regulus makes a choice to leave, and this choice has far - reaching consequences for both him and Sirius. It can also explore the theme of redemption. If Regulus has made some wrong choices, his leaving could be a step towards redeeming himself, either in his own eyes or in Sirius's eyes. Additionally, the theme of the impact of the wizarding war on personal relationships is quite common, as their situation may be a microcosm of the larger conflicts and divisions in the wizarding world.