I am pretty sure (this is just a guess) that the MC isn't the murder nor is anyone for that matter but, he did cause his father to seek suicide most likely due to father not wishing for the MC to look up to him or something of the sort or that something happened to the MC or something that the MC said that compelled the father to commit suicide; i.e the MC telling his father that people were telling him that he should quit high school and get a job to try and help his family. The sister doesn't seem like the type to not tell the authorities that the MC killed their father so this leads me to believe that rather then actually believing that the MC killed their father she just believes that the MC caused their fathers death. Though all this is my thoughts on it and he could just as easily be the killer or his sister, or his mother, or some random person.