If you read the RotJ novelization, it states clearly during the Obi-wan scene that Leia was taken to Alderaan by her mother, and that her mother lived there with her for a while, dying when Leia was a few years old.

In this context, Leia’s ability to remember her is suddenly much more plausible. It also says that when Vader turned to the dark side and joined the Emperor, he had no idea his wife was pregnant — indicating that his turn happened for quite different reasons. And before anyone tries to discount the validity of the novelization as a source, remember that it was based closely on the film script and contains quite a bit of dialog that was written by Kasdan and Lucas but never actually made the final cut of the movie.

So even though these details aren’t in the movie itself, they are nonetheless quite important to the backstory context the film-makers had in mind while working on it. I’ve been aware of them ever since reading the novelization when I was 9. I fully expected that the prequels would adhere to them, and was extremely disappointed when I realized the extent of lazy revisionism that was going on.