My Sister The Vampire Eight

My Sister the Vampire is a cheery urban fantasy series dealing with a middle school (and later high school) pair of identical twins Olivia and Ivy as they navigate secrets, family tensions, and strange happenings hand in hand. When the cheerful Olivia Abbot  transfers to Franklin Grove Middle School, she worries about fitting in. Many of the students are meat-eating, black-loving goths, while Olivia is a vegetarian pink-enthusiast. But Olivia doesn't let that stop her and tries out for the cheerleading squad, making some new friends (and rivalries) along the way. Ivy Vega  has lived in Franklin Grove almost her entire life, but things get weirder than usual when a strangely familiar new girl shows up in her life. Eventually the girls realize they're identical twins and hurry to make up for lost time. But Ivy has a looming secret and struggles more and more to keep it from Olivia: Ivy is a vampire!

The series was written in a fiction mill by multiple authors all writing under the pen name Sienna Mercer. Originally printed in the UK by Harper Collins, Scholastic later ran a reprint of the first 13 books with updated cover art and 6 of the books re-named. Unfortunately, the 13th book was re-named to have the same title as the 18th installment, which has added further confusion.