Twisted Sister of Mine: Book Five of the Overworld Chronicles - John Corwin

little sister-BIG TROUBLE Justin Slade is living on borrowed time. The vampling curse is gnawing away at his body and his sanity. Because the cure might lie in his special kind of magic, Justin goes to Arcane University in a desperate attempt to unlock his powers. When he runs into his little sister, Ivy, at the university, Justin realizes this might be his chance to convince her he's not evil. Even better, she might be able to cure him if her shape-shifting protector, Bigglesworth, doesn't eat him first. Save his sister. Save the world. Save his life. With the clock ticking toward a future as a blood-sucking zombie, Justin has to make every second count.