What do you mean? It was ESB when we found out that Vader was Luke’s father, meaning Ben was lying to him in Star Wars.
ESB could have meant many things…not [i] necessarily [/i] that Vader used to be the elder Skywalker. An alternate scenario is that booth Vader and Ben could have still been telling the truth (as far as they knew), but difference being that Ben was mistaken about who Luke’s biological father was.*
*a scenario which would also explain why neither he nor Yoda warned Luke about what Vader might claim, and the Emperor going along with Vader’s plan (i.e. the Emperor mistakenly thought Luke was Annikin’s son as well).