Who's Your Daddy?

Lena is currently trying to figure out her family tree. For the last 2 weeks we've been getting a lot of questions about who is whose mommy, which of the many children she loves are her cousins, and most importantly, how is it that I am both her mommy and Pilar's mommy. I think part of the reason for the questions is her California grandparents will be visiting in June and I am VERY excited and she's heard me talking about it to Jason. Oddly, she seems to understand the concept of two sets of grandparents (she often calls my parents her great grandma and great grandpa), but the whole 'your mommy is also Pilar's mommy' is totally blowing her mind. She had a tiny freak out yesterday about it. She kept yelling, "you are not Pilar's mommy you're MY mommy!" I'm letting it ride for now, since sooner or later she'll figure it out. The funniest part of all this is her theories of who begat whom. She has informed me several times that when she grows up she will be my mommy, grandma's mommy, and/or Pilar's mommy. Jason has been explaining that to be a mommy she has to create a whole new person, but that explanation has been rejected. I'm thinking there should be a children's book about this somewhere (there are children's books about pretty much everything) so I'm on a search. Any suggestions?