Murder Game birthday Theme?!?

That sounds super fun! What I would do is make it like you're a ghost, explain the rules of the game and divide your guests into like 5 teams of four. Then you could have your parents and even siblings as the suspects and your guests can interrogate them. Leave clues around and then the first team to correctly say who killed you how and where wins some sort of prize. I would make it like four hours... A couple hours for the mystery and a couple to do whatever. You could also modify for how smart or stupid your guests are.