Agreeing with QUIRO 72, 2 things in ep4 that made me think vader was always luke's father.
Aunt beru: "Luke's just not a farmer Owen, he has too much of his father in him"
Owen:" That's what I'm afraid of"
could be explained by anakin(not vader) dying as a jedi
but...
remember obi wan:
"He was the best starpilot in the galaxy..." looks like he remebers a battle or something kind of proud...
"and he was a cunning warrior"...has almost a little sneer or glare in his eye, like he knew damn well how good anakin could fight...
"and he was a good friend..." obi is sad, thoughtfull, but..DISTRACTED,
he looks away absent mindedly, and goes on about the lightsaber, in my opionion, because he has just remembered all of the events that we have seen take place, looks up and remembers Luke, which is the completion of his "quest", I suppose.
Also, when Luke asks how his father died, notice ben hesitates, just for a moment or so, takes a breath, and tells him that the young jedi named darth vader, who was his student, then turned to evil helped palps hunt down all jedi's. THEN he throws in the crap about he betrayed and murdered your father.
Even more important, the next line:"vader was seduced by the dark side of the force" Ben gets stuck again, and, again luke brings him out of it, to ask about what the force is. and you can see that ben kind of forces a smile, and starts to explain things to Luke.
Might just be me, but that scene had alot of meaning, IN 1977. 😄