Or, if he had wanted to remain consistant with what was said in Return of the Jedi, he could have made her give up Luke and go to live on Alderaan with Bail and his wife, then Leia could have remembered her real mother. Palpatine had no way of knowing she was really dead or not, everyone could have just said she was dead while she lived out the rest of her days hidden on Alderaan. But of course consistancy was hardly George's goal.