In this fanfiction, the seizure could be part of a curse that was meant for Sirius but backfired on Harry. Harry starts seizing in front of Sirius, and Sirius is filled with guilt and determination to reverse the curse. He uses his knowledge of the wizarding world and his connections to search for a cure, all while taking care of Harry and keeping him safe from any potential threats.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
One reason could be that Harry is under an extreme amount of stress. He has so many burdens on his shoulders in the wizarding world, like fighting Voldemort. This stress could manifest physically as a seizure, especially if he is in a tense situation in front of Snape, who is often a source of anxiety for him. For example, if Snape is about to accuse him of something he didn't do while he is already worried about other threats to the wizarding world.
Snape might initially be suspicious. He has always had a distrust of Harry, so he could think it's some sort of trick. But then, as a Potions master and a teacher, he might also feel a sense of responsibility and try to help.
Perhaps in this fanfiction, the bad seizure is a result of some internal struggle within Harry. He could be dealing with the immense pressure of his destiny, the weight of the prophecy, and all the losses he has endured, which manifests as a seizure.
It could be a new dark wizard or witch who has a vendetta against Harry. This person might have developed a new form of magic specifically to harm Harry, and the seizure is the first sign of this new threat.
A possible plot in this fanfiction could be that Harry moves in with Sirius after a particularly difficult year at Hogwarts. Sirius would be a mentor to Harry, helping him with his magic skills and dealing with his nightmares. They could also be on the lookout for any signs of Voldemort's return together, while Sirius tries to give Harry a more stable and loving home environment. Harry would grow in confidence and magical ability under Sirius' care.
Well, perhaps Sirius trains Harry in advanced defensive magic. Harry could learn spells like the Patronus charm more effectively under Sirius's tutelage. Sirius might also teach Harry about the wizarding world's secrets that Dumbledore didn't share.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.
Well, it's fairly common. Fanfic writers often use this as a means to create drama and tension. Since Sirius is such an important and mysterious figure in Harry's life, the idea of him kidnapping Harry can lead to all sorts of interesting storylines. It can also be a way to rewrite parts of the original story, perhaps to give Sirius more agency or to create a different kind of adventure for Harry.
One possible plot could be that Harry grows up with a stronger sense of family. Sirius and Remus would teach him about his parents more directly. He might be more confident in his magic earlier, as they could start his training sooner. Also, he'd likely have a different view of werewolves and Animagi from an early age.