His mother could send the howler. In the wizarding world, mothers can be quite fierce when it comes to family reputation and she might be unhappy with Sirius's actions or friends.
In such a fanfiction, Sirius could receive a howler that reveals some family secrets. The howler might be a way for his family to try and control him or bring him back into the fold. It could also lead to some internal conflict within Sirius as he has to deal with the public humiliation of the howler and what it says.
It might happen if James doesn't follow Harry's instructions or does something dangerous. Like when he experiments with forbidden magic or disregards Harry's warnings about certain magical creatures.
Snape could also be a possibility. If Harry did something that Snape considered a serious offense in Potions class, like using an ingredient in the wrong way that could have caused an explosion, Snape might send a howler to scold him. However, it would probably also be filled with his usual sarcasm and dislike for Harry.
One popular type of fanfiction about James Sirius Potter might be his adventures at Hogwarts. Writers could explore his relationships with his friends and family, like how he deals with the legacy of his famous father, Harry Potter. Maybe he gets into some mischief in the same way his father did, but also has to face new challenges specific to his own time at the school.
Often, when Sirius gets a howler from his mother in fanfiction, it triggers a series of events. It might make him rebel more, or it could cause him to reflect on his actions. Sometimes, it even sets up conflicts with other characters.
Well, Sirius could get really flustered. It could lead to some comical or tense moments depending on the context of the fanfiction. The howler might reveal something he doesn't want others to know.