Well since she already knows the difference from make believe and real, be honest with her... I do understand wanting them to be kids longer and using their imagination but for some kids it's just not that...
Have you ever asked her a question back instead of answering the questions, like what do you think?
I remember one night my son was upset that the tooth fairy forgot to leave $$ under his pillow the night before... He was in the 4th grade, and I thought he did know that it wasn't real, well, I told him, I'm sorry I forgot to do it last night... then he said why didn't the tooth fairy do it, and I said come on, you know I'm the tooth fairy, and with a surprise reaction he was like "you are" and I said, "come on, I know you don't still believe in the tooth fairy and Easter Bunny", then he says "There's No Easter Bunny Either"... and started to cry... I felt all bad inside, then about a couple weeks later, he said, you know, I was just mad at you for forgetting, I don't still believe in them, well, not really... I was so mad at him, but I also remember what it felt like to be a kid and wanting to believe... heck at Christmas I still want to believe there is a Santa out there...
See what she believes and go from there... You can always be honest and say, well I've never really seen one, but I believe they look like this, or I imagine they look like.... let her decide for herself if she wants to use her imagination or not...