The My Sister the Vampire Series
by Sienna Mercer (Author)
Olivia and Ivy are brimming with plans to switch places and pull every twin trick in the book. But Olivia soon discovers that she and Ivy aren't exactly the same. Ivy's a vampire. And she's not the only one in town.
Yes, Olivia and Ivy are the oddest set of identical twins you'll ever meet...just wait and see what they get up to!
Note: While this series doesn't need to be read in order, reading the first 4 or 5 books in sequence is helpful.