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.
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.
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.