I Would Die For You by Maya Shepherd

Six months ago, Winter’s sister Eliza disappeared without a trace. Nevertheless, their parents refuse to give up hope. They still believe that their eldest daughter will return soon. Winter never believed Eliza’s disappearance to be a crime. She and Eliza never got along and her sister never thought about anyone but herself.
On a school trip to London, Winter thinks that she sees her sister. Winter and her girlfriend Dairine start to investigate, and during this investigation their music teacher becomes a suspect. Suddenly, horrible ritual murders are happening in the region and Eliza becomes a suspect. Winter makes an observation which makes her doubt that her sister is even still human.