I lost eveything

Elizabeth Mill is very close with her younger brother. Many people make fun of them because of how close they are and call them sibling lovers. Elizabeth always preferred to be alone besides being with her close friend Megan. Daniel her brother dies because of her. She feels betrayed and heartbroken. All of it started when that boy came to her class. What did Megan have to do with this? What was that boy? and why did she lose the most important thing in her life?