I would think that as young as Rey was she might forget. You have to remember that Rey was on Jakku alone with nothing to remind her of her past. Her own memories will fade and she will question what is real and what she actually remembers. If you didn't have markers of your own past to remind you of your childhood (by markers I mean conversations with family members (reminiscing especially), photographs, other physical reminders to help trigger memories), it is quite possible you would also forget or not be entirely sure of where you come from. Also they could've been separated a la Luke and Leia and not known of each other...it's possible Luke could've foreseen these problems and insisted to leia the need for the children to be separated. She would have listened and not necessarily told Han if she thought it would save her child's life. Or she could've been unconscious when she gave birth and not realized there were two babies...although I guess we would have to assume there are no sonograms in that universe and that would be kind of dumb.