Harry Potter is famous for his temper, but what if his emotional outbursts weren’t entirely his idea? Harry’s godfather, Sirius Black probably pushed him into boiling over, with magical consequences.
One fan on Reddit suggests that at the beginning of Harry Potter and the Prisoner of Azkaban, when Harry blew up Vernon Dursely’s sister, Marge, it was actually Sirius’s doing. The evidence stacks up, too. For one, before the blow-up incident, Vernon and his sister had been implying that James Potter — who had been Sirius’s best friend — was an unemployed drunk. We know that Sirius would go to great lengths to enact vengeance upon those who insult his friends; he almost kills Severus Snape during the Marauder’s school years. For another, we know that Sirius was lurking outside on Privet Drive and listening to the Dursleys insult Harry’s parents because Harry sees him in his Animagus form after he runs away.
Here’s the relevant passage from The Prisoner of Azkaban.
A funny prickling on the back of his neck had made Harry feel he was being watched, but the street appeared to be deserted, and no lights shone from any of the large square houses … “Lumos,” Harry muttered, and a light appeared at the end of his wand, almost dazzling him. He held it high over his head, and the pebble-dashed walls of number two suddenly sparkled; the garage door gleamed, and between them Harry saw, quite distinctly, the hulking outline of something very big, with wide, gleaming eyes.
Sirius could have easily grown enraged to hear James insulted and used wandless magic. Letting Harry take the blame for it kills two birds with one stone for Sirius: He gets revenge on someone insulting James and he gets his godson out of a bad living situation.
Lastly, although it’s certainly plausible that Harry blew up Marjorie through his own uncontrolled magic, it would hardly be the first time the Harry Potter books presented a scenario in which there is confusion over the source of magic.
In Chamber of Secrets Dobby performs a Hover Charm that’s blamed on Harry and at the end of The Half-Blood Prince, Harry is able to join Dumbledore in the row boat that cave because it doesn’t register his underage magic. When Harry remarks that the boat looks built for just one Wizard, Dumbledore responds, “I rather think an enchantment will have been placed upon this boat so that only one wizard at a time will be able to sail in it. I do not think you will count, Harry: you are underage and unqualified.”
Sirius’s manipulation doesn’t imply that he doesn’t love his godson — far from it. It would be entirely in line with his daring and reckless brand of caring.