Killer in the school

You decide to find out who he is. You attempt to pull his hood off but he wakes up pushes you away slashing your arm. You scream in agony and run out of the room. You continue running until you reach the sign you thought you would never see. The library. You walk up to its doors and try to open them but they are locked. You are about to give up when a boy steps out of the shadows. It is Dan. "Dan I need to get into the library" you say to him. "Okay I can give you the keys to the library if you can answer my riddles". He points at a skeleton on the wall. "Brothers and Sisters I have none, but that mans father is my fathers son, who is he? Will you reply: