Mrs. Weasley ushers Harry, Ron, and Hermione upstairs. Harry and Ron climb into bed and Ron bolts the door so Kreacher doesn't wander in. They begin to talk about what they heard when Fred and George Apparate onto Ron's bed. They all discuss what the "weapon" might be and whether Dumbledore might have it hidden at Hogwarts. They wonder if it can cause pain or kill lots of people. Fred and George Disapparate when they hear Mrs. Weasley coming.
The fact that all of Harry's peers also fixate on the "weapon" as a weapon in the conventional sense, like a gun or a violent curse, suggests that thinking this way is something typical to the youth of the novel. They don't wonder if it's something like information, which can also be useful but isn't as flashy.
Harry dreams of creatures and weapons all night and wakes up to George saying to eat breakfast quickly so they can deal with the doxies in the house. A half-hour later, Harry and Ron join Mrs. Weasley, Hermione, Fred, George, and Ginny. They tie cloths over their noses and mouths and pick up bottles of Doxycide. Mrs. Weasley wonders out loud what Kreacher's been up to since the house is such a mess, which offends Hermione, but Sirius appears and insists that Kreacher is more than capable of keeping the house clean if he wanted to. Someone rings the doorbell downstairs and Sirius runs down to stop Mrs. Black's screams (she’s awakened by the sound). Harry takes as much time as possible closing the door and hears Kingsley say that he's come to leave a report for Dumbledore.
Notice that though Mrs. Weasley is a good person—she's kind, she's the closest thing Harry has to a mother, and she fights against Voldemort—she still isn't perfect, given that she speaks somewhat callously about Kreacher. As a house-elf, Kreacher doesn't have the best lot in life and is deserving of kindness and respect. This shows that this kind of discriminatory language and thought against house-elves is something ingrained in Wizarding society.
Mrs. Weasley explains that they'll all spray at the curtains, which will paralyze the doxies, and then they'll throw them in buckets. Harry notices Fred pocket a doxy and George whispers that they want to experiment with their venom for their Skiving Snackboxes, a product they’re developing involving double-ended sweets. One end makes a person ill enough to get out of class, while the other end instantly restores them to wellness. The twins explain that they're currently running their business, Weasley's Wizard Wheezes, as a mail-order service out of the Daily Prophet.
Readers will recall doxies from previous books when the children learned about them at Hogwarts. The twins again show their great ingenuity, despite the fact that they’re not successful at traditional academics.
When the curtains finally stop buzzing with doxies, someone rings the doorbell. Mrs. Weasley runs downstairs to deal with Mrs. Black as the children look outside and see Mundungus with a stack of cauldrons. They figure he wants to hide them at Grimmauld Place, and this is confirmed when Mrs. Weasley starts yelling that Mundungus can't store stolen goods here. George starts to shut the door but the house-elf Kreacher slides into the room first. Kreacher ignores everyone and mutters to himself about how Mundungus and everyone else are destroying his mistress's house. He also calls Hermione a “Mudblood.” When Sirius steps into the room, Kreacher insists that he's cleaning.
The way that Kreacher speaks about Hermione and Mundungus indicates that he bought into Mrs. Black's view of the world entirely, even as it dehumanizes him and denies him any power or agency. This reminds the reader that even among individuals like Kreacher who are harmed by discriminatory policies or regimes, there can still be impetus to go along with or support those regimes.
Sirius accuses Kreacher of trying to sneak things away so Sirius can't throw them out and sends Kreacher out of the room. Hermione suggests that they set Kreacher free, but Sirius curtly says that Kreacher knows too much. They approach the tapestry of the Black family tree that Kreacher was eyeing. Sirius points to where he used to be; Mrs. Black burned him off when he ran away from home at age sixteen. Sirius then points to his brother, Regulus, who died fifteen years ago. Sirius explains that Regulus joined the Death Eaters and their parents were thrilled, though they weren't Death Eaters themselves. They thought Voldemort had the right idea about blood purity.
The political leanings of Sirius's parents show again that one doesn't need to actually be a Death Eater to have a questionable moral compass. While Mrs. Black seemed to put a lot of stock in having the family tree written out and burning people out of it when they disappointed her, that clearly didn't work when it came to allocating things after her death—Sirius still got her house. Erasing someone from a family tree doesn't mean they're not family, with the rights and responsibilities that go along with that.
Mrs. Weasley returns with a plate of sandwiches, but Harry stays with Sirius at the tapestry. Sirius remarks that Tonks isn't on the family tree anymore, either, as her mother married a Muggle. Tonks's aunts are Bellatrix Lestrange and Narcissa Malfoy, which shocks Harry, but Sirius explains that all pure-blood families are related. He says he's even related to the Weasleys. Harry is stunned that Sirius and Bellatrix are related, but Sirius snaps that she's not really family. He apologizes and says he never thought he'd be stuck in this house again, a sentiment that Harry understands. Sirius then suggests that he accompany Harry to his hearing in his dog form, but remembering the hearing makes Harry's mood plummet again.
Just as erasing someone off the tree doesn't alter what family is in a legal sense, being on the tree doesn't mean that Sirius has to think of these people as family either. This suggests that one's actions are more important than blood, given that Sirius relies on his friends to give him the support that family might have shown him, as does Harry. Similarly, Tonks's mother was taken off the tree because she presumably married someone she loved, further supporting that love and actions are more important than blood.
Harry asks if he can come live with Sirius if he's expelled, but Sirius won't promise anything. For the rest of the afternoon, Mrs. Weasley and Sirius lead the effort to clear off shelves filled with magical objects. Several of the objects try to harm them, and Fred and George steal a few. They continue to clean for a number of days, and Harry and his friends have little luck listening in on conversations downstairs. Harry feels fine as long as he's busy, but in quiet moments, he fixates on the hearing. The night before his hearing, Mrs. Weasley explains that Mr. Weasley will take Harry to work with him. Harry is hurt to learn that Dumbledore was at Grimmauld Place the night before and didn't bother to speak to Harry.
When Harry's life turns into cleaning and decontaminating the house, it impresses upon him that the kind of war the Order of the Phoenix is waging is mostly pretty boring—and though Harry might not realize it, the work he's doing is what allows the Order to get on with its other work that tackles Voldemort directly. This reinforces the importance of domestic labor to resistance efforts like this and shows that those efforts cannot exist without all the people who feed, clean, and otherwise support the lives of the members.