Small smile. "And we can add Legilimency and discerning true memories from false to your summer projects, if you'd like - it'll help your own Occlumency, if nothing else."
She tells Sirius to stay in the living room with Scarlet, then leads Anathema and Ellie into her smaller, private office. There's several bottles with odd, swirling lights in them, and a strong-smelling potion in a glassy stone bowl. Professor Reynolds idly describes what's what - the bottles contain several of Sirius's memories, checked for authenticity and kept separate. The stone bowl is a pensieve. The potion is specifically for clearing psychic residues from a pensieve between uses. She walks both girls through responsible pensieve use, and how to pull yourself back out, and explains that the contents of the memories may be distressing.
She pulls memories out of her own head with a twist of her wand, after they get through Sirius's, though she puts them back into her head, while Sirius's return to their bottles.
The parade of memories paints - a disturbing picture. Many of Sirius's memories seem damaged, almost, the color faded, parts worn thin - usually a pensieve memory allows you to observe things the originator didn't notice, but most of those parts look like they were rubbed out of Sirius's head. Exactly typical of long-term exposure to Dementors, Professor Reynolds explains, which is hard enough to mimic it makes the memories seem more likely to be true. False memories are also almost immediately destroyed by Dementors, making it very unlikely any alterations date to before Azkaban.
Sirius was James and Lily's best friend - he seems familiar in their house, with them, in the discussions shown. All three look stressed in the memories - Lily pregnant, speaking about a prophecy - Lily with an infant Ellie in her arms, standing and pacing and bouncing her, worrying if the Fidelius will be enough, with how often Sirius is putting himself at risk of capture - with how vital he is to the war effort.
That discussion goes on for a bit. James suggests changing the Secret Keeper. Sirius balks. Lily wonders 'to who.' They shoot down Dumbledore immediately. They shoot down Remus, James noting he's been reclusive, lately, objecting to the treatment of werewolves as the war's been spiraling - James seems suspicious, Sirius sides with Remus, says of course the laws are ridiculous, not all werewolves are Greyback - Lily ends the argument, pointing out that Remus is risky for similar reasons to Sirius, anyways. Too exposed. Too vulnerable. They shoot down a few other names - Fay, Kingsley, Minerva - for the same reason they're hesitant for Sirius. Peter Pettigrew is Lily's idea - 'I hardly think anyone remembers you were friends in school, but you all always seemed so inseparable...' James is enthused. Sirius is still insisting he can keep himself out of Death Eater hands - 'I'll damn well suicide before I let them take me, and you two can hide in a forgotten house until the war's over.' James objects; the argument fades, skims forwards, to a snatch of conversation with a short, twitchy man - Peter - as he promises to be the a trustworthy Secret Keeper, he never goes near the fights anyways, he's in logistics so he's not at risk -
Flash skim over Sirius learning of the Potters' deaths. Flash skim, confronting Pettigrew. Pettigrew turning to him, immediately accusing him of betraying their friends - Sirius doesn't even have his wand up as Pettigrew blows up the crowded street -
Flash skim. An earlier point, Sirius much younger, standing with James and Peter and Remus, showing clearly Peter shifting to rat form, Sirius shifting to dog form, James to a large stag -
Pettigrew's memories. He'd been Voldemort's man since before accepting the Secret Keeper position. A mole, slipping reports on funding and provisions and the like to Voldemort - nothing vital, of course, he told himself, he always told himself, it wasn't like he was relaying troop movements or bases - his intelligence of course unattached to the Death Eater's repeated successes in hunting down the Order of the Phoenix, in finding their identities and murdering their families -
He hesitated, after being made Secret Keeper. He learned - Voldemort had received a prophecy, about a newborn destined to eventually defeat him, and wished to nip the problem in the bud - prophecies name but one world, he'd said, and that one will be destroyed in its cradle. Voldemort promised a handsome reward to anyone who revealed one of several families to him, all with recently born children who might fit the prophecy. He followed through - he killed each family and child personally, and the riches rolled in to his informants -
Pettigrew flipped on the Potters, the most well hidden of Voldemort's enemies. Voldemort had been closing on the Longbottoms. He decided to go after the Potters instead, with such a ripe opportunity.
Pettigrew knew he screwed up, when he heard Voldemort had died. He knew he'd really screwed up, when Sirius caught up to him. So he did what he's always done - he lied and hid, away from any threat of retaliation, making his way eventually into a wizarding family's keeping as their pet rat. A flash - he'd been considering trying to probably resurrect Voldemort after he heard rumors about Professor Quirrel. He'd been very sure being a rat was miserable, and he wanted something more - the reward for not only the best information, but the most faithful servant -
He'd been seriously calculating whether he should make a run out of Hogwarts before winter break, or if he should abandon the wizarding family when the kids went home and flee from there.