Where was I when this happened?

DD and I were looking at our wedding pictures. Nothing foreshadowed trouble. DD got very cute and bubbly and said, "Mommy, you looked like a princess!" All of a sudden, I hear a question, "Mama, where were DB (dear brother) and I?"
Me: (sensing serious trouble ahead) Um, you weren't born yet.
DD: We were home?
Me: No, honey, you weren't here yet.
DD: Why did you leave us home? I want to go there (i.e. our wedding).
Me: (pondering on how to explain the impossibility of time travel to a toddler - or is she pre-schooler already? - and the fact that there was a time when she didn't exist.) I don't think it's possible.
DD: Why didn't you take us with you?
Me: (lightbulb moment). We didn't leave you home. (She is skeptically nodding and saying ya-ha!) If we did, who babysat you? Grandpa and grandma were also here! And so were your aunt and uncle! And you see, two of your cousins are not in pictures either! That's because they weren't born yet, just like you!

DD was stumbled and left me alone for a while. Then SubHub came home, and she decided to try him.

DD: Papa, why did you and Mommy get married?
DH: (stumbled. Really, why?) Ummm....
Me: (coming to the rescue) so that Papa and I could have you and your brother. (whispering from the corner of my mouth to SubHub) and I'll talk to YOU later... (Then I retell him the conversation we had yesterday and that DD finds it hard to believe that we got married (or anything existed) before she was born.)
DH: yes, mommy and I are married for five years, you just turned four, and DS is two and a half. So you see, first we got married, and then you were born.
DD: Why did you get married first?
DH: (a bit exasperated) because if we did it in the opposite order, Daddy's rabbi would be very, very upset...

Somehow that resolved the issue, but I am sure we haven't heard the last of it. How do you explain to a 4-yr old (did I mention she turned four?) that there was time when she didn't exist? I think I am still wrestling with the concept...