marriage has the kid confounded

The other day, on our way home from preschool, Elisabeth wanted to hear her favorite song these days, Bibbidi Bobbidi Boo from Cinderella. This got her chatting about the wicked step mother.
E: "Why the step mother is mean mommy?"
Me: "She doesn't like Cinderella."
E: Why not?
Me: I don't know.
E: She likes Anastasia and Druzilla, right mommy?
Me: Yes, they're her daughters.
E: Cinderella isn't her daughter?
Me: No, she is Cinderella's step mother.
E: What's a step mother?

I should have feigned ignorance on this point. This is not an easy subject to explain to a three year old. But since Doug has a step mother who is decidedly not an evil step mother (quite the opposite in fact), I wanted to dispel any notions that evil must always precede "step mother." So I said...

Me: A step mother is when your daddy isn't married to your mommy, and instead is married to someone else.
E: You're married to daddy. You listened to Allison Krauss when you got married. And I stayed home and played with Becky. (the babysitter, she is confused about anniversaries and weddings.)
Me: Right, I'm not your step mother, I'm your mother.
E: Oh.
E: I'm going to marry Charlotte.
Me: Oh, why?
E: Because she's my besssssst friend.
Me (relieved, thinking the step mother conversation had satisfied her and I don't have to try to explain this anymore...): Oh, that is so sweet.

blissful peace for 30 seconds.

E: What's a step mother?
Me: ok. you know how Pop Pop - daddy's daddy - isn't married to Gram? But is married to Nana? Well Nana is daddy's step mother.
E: gasp! Nana is a step mother? OH I am so happy! (she loves Cinderella).
E: (ponders this new concept)
E: So, are you my step mother?
Me: no, I'm your mother. A step mother is someone who is not your mommy.
E: I'm not your mommy - so I'm your step mother!
Me, in utter frustration: No. You're my little girl. If Daddy ever married someone else, she'd be your step mother.
E: Daddy's getting marrrrrrried???
Me: No. Daddy is already married. To me. And I'm your mother, not your step mother.
E: oh. Nana is nice, though.
Me, happy that the entire conversation has not been for naught: Yes. Nana is nice. Cinderella's step mother is not nice, but Nana is very nice.

I think just maybe she was pulling my leg throughout some of that conversation because she is not usually so exasperating.

Then this morning, she got Doug's wedding ring and said, "Daddy, here's your work ring." (He only wears it when he leaves the house, which is essentially only when he goes to work). He told her that it is his wedding ring so people know that he's married to mommy. She put it on and started explaining that she was going to wear it so that everyone knows she's married to Charlotte.