Complex sentence: Andy's first girlfriend was his classmate Katie Addyman but he was unaware that his adoptive brother Robert knew that he was responsible for Sarah's death and wanted revenge, particularly after Andy teased him about not having a girlfriend.
Split into simpler sentences: Andy's first girlfriend was his classmate Katie Addyman but he was unaware that his adopted brother Robert knew that he was responsible for Sarah's death and vowed revenge. Particularly after Andy teased him about not having a girlfriend.