Marked for Death by Rebecca Hamilton
There's a magical cure for cancer, and it lies in the last drop of Acadia's blood. Acadia's sister has just been diagnosed with cancer. Living in foster care, she's already lost everyone else in her family; she's not about to lose her sister, too. But when a mysterious man shows up promising magical answers, Acadia never expects the price to save her sister's life will be her own. Acadia blood has the answers- ingrained in her dormant magical abilities--but the cure will only be revealed when the last drop of blood spills. Acadia's ready to sacrifice her life if it means saving her sister's--until she learns that there's a darkness in her blood as well. A darkness that, once her blood spills, will infect the world and allow the uprising of a secret demon clan.