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.
His father, Harry Potter, might also send a Howler. Harry knows the importance of discipline and responsibility, and if James was shirking his duties or not following the rules at Hogwarts, Harry would not hesitate to use a Howler to communicate his disappointment and expectations.
One possible plot could be that James Sirius Potter has done something really naughty at Hogwarts, like pulling a major prank that went too far. The Howler then arrives, screaming at him in front of all his friends and classmates, causing him great embarrassment. This could lead to him having to face the consequences and also learning a valuable lesson about the limits of his mischief.
Perhaps Sirius is scolding Harry for getting involved in something risky, like sneaking into the Forbidden Forest. He might be worried about Harry's safety and uses the howler to convey his strong emotions. In the fanfiction world, Sirius could also be using the howler to give Harry some crucial information about his family that he has just discovered and wants Harry to know right away.
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.
Well, it could be scolding him for his association with the 'wrong' sort of wizards, like the Potter family perhaps. His mother might be furious about his choices that go against the pure - blood supremacist views of the Black family.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.