She Runs with the Bad Guys - The Runningwolf Trilogy, book one

Hermione Jean Granger, mudblood, Griffindor Princess, and the Bookworm… Or is she? When Lord Voldemort first came to power, the Runningwolf family faked their deaths, causing everyone to think that their Pureblood lineage was destroyed. But they’ve hidden in the muggle world, protecting their three daughters, one of which is Hermione. A true hearted Slytherin. With the resorting and everything, she’s beginning to show her true colours, and it makes a certain blond haired Slytherin change his opinion on her… (Reminder: Takes place during seventh year)