George Lucas didn't plan to have Darth Vader be Luke Skywalker's father when Episode IV came out. But he worked it in and the idea makes sense from a certain point of view--as Obi-Wan later tells Luke.
It gets more complicated when Episodes I-III describe the rise and fall of Anakin Skywalker. Some fans think that it doesn't make any sense at all. But YouTube user Shahan Reviews edited Obi-Wan's remembrance of Luke's father so that he seems haunted by the past. Obi-Wan isn't lying to Luke. He's just trying to come to grips with what he lost those many years ago.
-via Dorkly