A Change in Circumstances

When Holly Potter is 5, she finds out that she's in fact not a Potter at all, but Regulus Black's biological daughter, thanks to a letter her mother left for her. She starts to learn magic on her own and, after having being able to prove Sirius' innocence, she goes to Hogwarts not as the girl-who-lived but as Electra Black where she's sorted into Ravenclaw. AU, HP/SS later on.