What are you talking about? Obi-Wan shows up as a Force ghost after Yoda dies and helps Luke realize Leia is his sister. Then, before leaving the Ewok story time to surrender to Vader, Luke tells Leia that she's his sister.
Then she tells Han after the battle of Endor is over. It's pretty clear how that info was generated and passed between characters in a non-story busting way.
That's....an extremely important part of Return of the Jedi. It's like saying you don't remember the part where Darth Vader reveals that he's Luke's father.