The Selwyns and the Weasleys

Carolyn and Emily Selwyn are two daughters of an ancient pureblood family. But instead of following in their parents' footsteps, they followed the ones of the "mass murderer" Sirius Black; even though they don't know him. Carolyn's aim is to protect her sister, no matter what. And a well-known Weasley will help her, along with protecting the girl he loves.