Chapter 6 - "The Noble and Most Ancient House of Black" - In which Harry, Sirius, and the Weasleys clean the Black house; Kreacher, the Black family house-elf, shows another darker side to house-elves; Harry is shown the Black family tree; and we learn that Sirius is not only related to the Weasleys and Tonks, but to the Malfoys and the Lestranges.
The Mission of the Order of the Phoenix
Thanks to Sirius and Lupin, Harry learns that Voldemort has been in hiding, rounding up followers, and that the [Ministry of Magic] is terrified of [Dumbledore] and determined not to let anybody believe him. Finally Molly stops the conversation, Lupin agrees, and everyone heads off to bed.
Mrs. Weasley puts Ron and Harry to bed, where they are soon joined by Fred and George, who [Apparate] in. Together they wonder aloud about the weapon that Voldemort is after, and quickly quiet down when they realize Mrs. Weasley is listening outside the door.
The morning begins with Harry, Hermione, Sirius, and the Weasleys attacking an infestation of [doxies] - and Fred and George making off with several - and Molly runs downstairs to grab lunch for everyone.
Ignoring everyone in the room, Kreacher arrives, muttering about the filth that has entered his house. He sneaks about the room for a bit, ignoring Hermione's attempts to be friendly, until Sirius kicks him out. Hermione then suggests setting him free, but Sirius explains that he can't do so.
The Black Family Heirlooms
The group spends the afternoon cleaning out the [Black family]'s glass cabinets, with Sirius doing his best to throw everything away and Fred and George doing their best to save whatever they can for their joke shop.