Obi was indeed lying. Not to be jerk but to safeguard the galaxy from yet another hot-head Skywalker. In ANH Luke is all about joining "the Academy" whose graduating classes no doubt head straight for service in the Empire. Why? Because his dad was a pilot. If Luke was told at that time that Vader was his father, he would have made a beeline right to him. He either wouldn't believe his father was an evil overlord or couldn't grasp the concept as a farmboy in the middle of nowhere. Either way he would be knocking on daddy's door and in a much better position to turn to the darkside. Obi wan knew this to be true.