So you are proposing that the backstory should have gone something like this: Vader conceives two children, but they are both taken away before he learns about their existence – the boy is placed with someone named Skywalker, who wants to raise him as his own, but is soon murdered by Vader. I like it a lot, but it would really strengthen Luke’s motivation to go bad: not only did they blatantly lie to him, they even kidnapped him from his real family.
Well, again the twin plot and the abduction at birth is only contained in ROTJ. It doesn’t change anything to Luke, he still has been kidnapped from his real family (in Lucas canon, Owen and Beru are not bloodrelated to Luke).