Vader wasn't Luke's father at first. In Leigh Brackett's first script draft, Luke meets his real dad, who says he sent away Luke and his secret sister for their own safety. (Luke's sister has been training to be a Jedi knight in secret, just as Luke has.) And Papa Skywalker administers the oath of a Jedi Knight to Luke, in which Ben, Minch, Anakin and Luke cross lightsabers, and Luke swears to "dedicate my life to the cause of freedom and justice."