Star Wars Episode 6: Return of the Jedi - I Remember Mama?
In Return of the Jedi Luke asks Leia if she knew her birth mother. She responds in the affirmative, then goes on to describe her. However in Revenge of the Sith Padime dies hours after giving birth and much too early for either child to have had any memory of her.
It is possible for Leah to remember her mother. Even though she wasn't a Jedi, she still able to use the force, as seen in "The Empire Strikes Back". All Leah said was "she remembered she was sad", and this was the same nightmare that Anakin was having during the movie
This is not a slip-up, it was a conscious choice made by Georges Lucas when he wrote the story. The idea that Leia remembers her mother is true or not true, depending on whether or not you look at it from, as Obi-wan might say, "a certain point of view". The real flaw you should question is, if she was separated from Luke, hidden from the Emperor and Vader, and sent to live as the daughter of Bail Oragana for her protection, why would Bail then reveal to her that she was adopted? If you follow that first question, it opens a very large can of worms.