The next morning, Harry goes down to breakfast and finds Aunt Petunia, Uncle Vernon, and Dudley watching TV. Harry sits and helps himself to toast as a newscaster warns viewers to be on the lookout for Sirius Black, an escaped criminal. Vernon snorts that Black is a "filthy layabout" and looks at Harry, whose hair refuses to behave but is still, in Harry's opinion, neater than Black's. The reporter turns to a new story, which angers Vernon—he shouts that hanging is the only way to deal with criminals like that.
Uncle Vernon's assessment of Sirius Black shows that what he thinks of the criminal is heavily influenced by the way the muggle news portrays Black: Harry will soon learn that Black is a dangerous wizard criminal, and that his unkempt appearance is the last thing anyone should worry about when it comes to Black.
Uncle Vernon announces that he needs to go pick up Aunt Marge. Startled, Harry asks if Marge is truly coming. She's been horrible to him since he was little. Vernon snarls that she'll be visiting for a week and tells Harry that he must behave and keep his "abnormality" a secret. Then, Vernon says that he's told Marge that Harry attends St. Brutus's Secure Center for Incurably Criminal Boys. Harry, furious, feels that this is the worst birthday present ever. Petunia fusses over Dudley as Vernon heads for the door, but Harry has an idea. He follows Vernon to the hallway and brings up the Hogsmeade permission form. Harry says that if Vernon signs the form, he'll remember the story and play along. Panicked, Vernon agrees.
Uncle Vernon's fear of Harry and of magic in general encourages him to spin this ridiculous story for Aunt Marge, which suggests that feeling strong emotions can influence how a person chooses to portray facts. As Marge is a muggle, it's worth noting that were Harry to tell the truth, it would make Harry and possibly Vernon himself look ridiculous, which shows that Vernon has a lot at stake in making sure that his preferred story is the one that, as far as Marge is concerned, becomes "true."
Harry heads upstairs and morosely packs up his birthday haul to hide with his spellbooks. Then, he tells Hedwig to go stay with the Weasleys; she's likely to give him away if she stays. Not long after he puts Hedwig's cage away, Aunt Petunia yells for Harry to come downstairs. When they hear Uncle Vernon and Marge arrive, Harry opens the door. Marge, her bulldog Ripper tucked under her arm, goes straight for Dudley. Dudley escapes from the hug with money as Marge kisses Petunia. Harry takes Marge's suitcase upstairs as the rest of the family goes to the kitchen for tea.
Harry's desire to properly play the part of a criminal muggle boy shows just how badly he wants to earn the signature to go to Hogsmeade. For him, allowing his enrollment at St. Brutus's to be true means that he'll be able to go on to live a happier life at school. This indicates that there are times when lying or spinning tales like this has positive effects.
When Harry finally enters the kitchen, Ripper growls at him and Marge confirms with a grimace that Harry still lives here. He remembers the Hogsmeade form, forces a grin, and as the conversation turns to St. Brutus's, he decides to play it up and tells Marge that they beat him all the time. She's not impressed. Throughout her visit, Marge torments Harry and insults his parents. At one point, her wine glass explodes and Harry excuses himself—he lost control and knows that if he keeps on accidentally destroying things, he could be expelled.
Harry was threatened with expulsion in the second novel if he did magic outside of school again. His fear here suggests that he could easily be expelled even for accidents like this, which begins to create the sense that the Ministry of Magic isn't very understanding: they'll persecute anyone, even if that anyone is a teenager who made a mistake.
For the next few days, Harry thinks of his broom care handbook whenever Aunt Marge is mean to him. On her last night, they get all the way through dinner before Marge starts in on Harry. After a large glass of brandy, she compliments Dudley's "healthy" size and then calls Harry "runty." She insults Harry's parents and when Uncle Vernon says that James Potter was unemployed, Harry loses his temper. He tells her that his parents didn't die in a car crash and as Marge swells with fury, she starts to literally swell. She rises out of her chair and bounces off the ceiling, and as Vernon tries to pull her down, Ripper bites his leg.
When Harry makes Marge swell up because of his anger, it indicates that Vernon's lie and Marge's rudeness are too much for Harry to bear—for him, it's far more important to tell the truth and deal with the consequences. The fact that Marge feels entitled to talk this way about Harry at all indicates that the story she believes is true gives her a great deal of power, while Harry's status as a child means he has little power to stand up to her.
Harry races away from the debacle. The door of the cupboard under the stairs flies open as he reaches it, and he heaves his trunk to the front door. As he fetches his things from upstairs, Uncle Vernon accosts Harry. He yells at Harry to fix Aunt Marge, but Harry refuses. His wand pointed at Vernon, he drags his trunk out into the street and starts to walk.
Vernon's request that Harry fix Marge indicates how little he knows about the wizarding world: it's unlikely that Harry, with only two years of education under his belt, has the skills necessary to put her right even if he agreed to try.