Everyone is related to someone and then they die

E has been interested in relationships for a long time. She loves to repeat that "Yaya is Daddy's mommy" and that "Gramps is Mommy's daddy". In conjunction with the death talk, she has realized that Yaya and Papous had mommies and daddies who are now dead. She talks about this a lot and is very interested in how it all works. She asks if they miss them. Tonight she asked if they died when they became a mommy and daddy. They explained that it was a long time after they had kids.

Then she said something she has been saying these past three days, "Mommy isn't my mommy. I have a mommy named Fredrick. Fredrick lives in California." I suggested that she was pretending and told her I would pretend as well. She shook her head and said "I am not pretending! Fredrick is my real mom." I wasn't sure what to say since she usually agrees that things are pretend and we play from there. She asked me a few times to ask her who her mom was so she could report "Fredrick". She said her dad is named "blah dooo goo dahh loo". I am not sure how to respond. I tried giving her a big hug and saying that I loved her so much and would always be her mom no matter what but she just told me I wasn't her mom. I am pretty sure it is related to the death issue and possibly the fact that we foster.

Any advice? Do I play along? Do I continue to reassure that I am her mom? We aren't worried about her knowing about death. I am concerned about her worrying about it too much though. Thanks for your insight!