Easterhay said:

Obi-Wan is lying to Luke about his father wanting him to have the lightsabre. Perfectly understandable in the context of what we know now. And isn't the way Alec Guinnes paused before replying to the question "How did my father die?" just perfect? It's as though Lucas had told him the backstory on the quiet.

I call bullshit on this. When star wars was written darth vader was not Lukes father and Obi Wan indeed told the truth. Empire was the first of many retcons to the saga. In the original backstory kenobi had two pupils Skywalker and Vader.