DISCLAIMER: ALL OF THE CHARACTERS AND SCENARIOS BELONG TO JKR AND/OR WARNER BROS.
Cecilia Frobisher is not where she had been.
She had been happily unmarried to Remus Lupin and had been living quietly in their cottage in the Lake District waiting for their time to get wed. She had helped the Order of the Phoenix to formulate a potion with a scientific foundation in collaboration with Severus Snape for Harry Potter in order for him to successfully fight and defeat Lord Voldermort.
It had been successful, but not as successful as everyone had hoped. Continuing her work Cecilia had been in the process of refining what she and Snape had been working on in light of a genetic influence of magic upon wizards and had, in addition and quite unexpectedly, formulated a possible scientific hypothesis as to why werewolves are adversely influenced by the full moon and why the "Wolfsbane" potion works to a certain extent.
For his part as a Recipricator, Albus Dumbledore had been put on trial in the Ministry of Magic, had been told to "oblivate" Cecilia's memory and not to operate as a Reciprocator again. When he refuses, dementors are sent to Cecilia at the cottage but before they had arrived Snape appeared unexpectedly and had made off with her in the nick of time as Death Eaters surrounded the cottage.
Tabitha Penwright, who now worked in the Ministry for Magic under Dolores Umbrage, had, later than most, discovered she was a witch, but quite a bad one. She had a gift for interpreting magical artefacts, a Mysteriour, so Tabitha's job in the Department of Mysteries suited her perfectly. Her brother however, a muggle, had been contacted by Death Eaters and Cecilia's friend, Libby and her husband, Derek had been murdered by him, leaving Freya, their daughter an orphan who Tonks and Nick Smith (a muggle scientist) had taken in.
Meanwhile Cecilia worked undercover as a tutor for none other than Dudley Dursley, whose new best friend, Darren Malloy, bore a remarkable resemblance to a white-haired young wizard we all know (and Harry despises) and who has been sent by his family to Dudley's school in order to undercover what he could from Dudley. It was only a matter of time before he discovered Cecilia's true identity.
On doing so, Cecilia, having figured out that Petunia Dursley had also magical gifts, but had turned her back on them because of her feelings towards how her parents had treated Lily more favourably to her and, on receiving a Hogwarts letter later on in her teens, had rejected it, having had a brief liaison with Regulus Lupin and thus calling in Dudley's paternity into question.
Cecilia fled to London, having encountered a Dementor along the way, which robbed her of her and Remus's child and, after a time in hospital arrived at Grimmauld Place and to Sirius. Remus, under Dolores Umbrage's new powers to eliminate halfbreeds had been arrested (as well as her tyrannical powers at Hogwarts). Sirius has a plan that involves going behind the veil at the Department of Mysteries, to change a memory and thus, the future. Cecilia insists she is the one to go.
By interacting with them memories behind the veil Cecilia had managed to stop Joseph Black from decrying the role of the Reciprocator as a link between wizard and muggle as illegal; stop Voldermort from therefore arising and therefore Harry not losing his parents and stopping the werewolf, Remus is not a werewolf anymore as she had thrown herself into Fenrir Greyback's path between him and her beloved.
When Cecilia was pulled from behind the veil, beyond was a world that she recognised but with obvious mistakes. Caelius Lupin, Remus's brother not only lived but was a senior ministry figure. Aberforth Dumbledore, not Albus, held the esteemed position as both headmaster of Hogwarts and head of the Reciprocators, not quite a secret society (as they interacted with and liaised between muggles and wizards) and whose members were not dissimilar to those of the Order of the Phoenix.
Severus Snape is now not a Death Eater and is living with Tabitha Penwright. Harry has a younger brother, Sam. Aberforth knows a little of Cecilia's history and where she has come from.
Caelius Lupin and Aberforth Dumbledore fear that the action of Albus and his partner Grindelwald would soon be a problem that they needed to challenge head-on and soon.
Cecilia fell in love with Remus in time (it had to happen!), though he is not quite the same as the Remus from the…other side…(as he is no longer a werewolf) and it is the consequences of this which begin the events of this story…