Vivianna Potter, the Slytherin Princess: Volume I

When Vivianna Potter, twin sister of the Boy-Who-Lived, enters Hogwarts School of Witchcraft and Wizardry for the first time, she sets off a string of improbable coincidences. She acquires allies, friends and enemies along the way, and in the end, must make an impossible decision.
Mostly, however, she is content to wreak havoc, though perhaps not quite the way her family imagines.