Maybe it's some Coraline shit going on... (I dunno I never saw the movie I just know the gist of it.)

That sucks more dick than her other mommy.

I used to call my Great Aunt "Mom" all the time and she very rarely corrected me (she watched me during the day). When I stayed with her overnight (sometimes a few nights depending on the weather and if Mom had to work overtime (Dad can burn cold cereal and has, so he didn't get "overnight childcare duties" very often) and it made it fun for me because she has grandchildren about my age), she was "Mom" (still Grandma to the grandkids), my Great Uncle was "Dad" (still Grandpa to the grandkids), their son (my cousin) was my "Uncle" (he is the grandkids's uncle, so I guess that is where that came from), their daughter (cousin again, and the grandkids's mother) was my "Aunt"(or "Mom" sometimes), I didn't see their dad very often (work schedule) so I didn't have a different name for him. He was just called by his first name, and my cousins (the grandkids who were my age), were my brother and sister. I also called the lady my "uncle" dated for a while "aunt" (and got a lot of strange looks) when she brought her son (who was also about my age) to play whiffle ball with me. I have also called my neighbors "mom" and "dad" (and the kids were "brothers" and sisters" if they had any, and even the animals were "brothers" and "sisters", no matter who they belonged to). I did that until I was almost 11, and still consider my Great Aunt an "almost Mom". As a car show kid (my parents went and still go to car shows every weekend from Mid-April? until Mid-October), I have a lot of "parents" from that (including the DJ). No matter what, I couldn't get away with anything (I didn't really try either), and I was always safe (or at least as safe as possible when playing on playground equipment), though Mom was never that far away. I remember falling once (from the top, of course) and landing hard on my face in mulch (even better), and it practically being a mob scene as all my "Moms" were trying to treat all the scratches and make sure I was ok. My "Mom" Mom (the real one) had to practically fight to just get a spot, then take me to the bathroom to rinse the mulch out of my mouth before I threw up, though the last part was a little late. (Mulch doesn't taste good by the way.) Oops (and ouch). I still consider them family and "almost Moms" too (though since it is old cars and old car show folks, most have past away or aren't well enough to come anymore).

At least she didn't scream it in the middle of a store--once when my mom wouldn't give my sister candy, my sister said something like "I WANT MY OTHER MOMMY" and it was the 90s before amber alert and before lesbians were more widely accepted, so my mom got a ton of dirty looks but no one did anything else.

Probably ask her what she means by that. Maybe she's talking about her grandmother, or her aunt, and just doesn't know the right word for it because she's five.

Just don't be too hasty. Some time ago there was similar FML... which was something along the lines "My daughter told me she liked other mommy more, so I confronted my husband about affair, he denied everything but I couldn't believe it, now he wants a divorce because I kept accusing him" and little angel of daughter lied because they didn't get something to her. Be skeptical, check every possibility, is your daughter angry on you for some reason, does your husband stays late, goes out more than usual, and do not confront either angrily, if you do, before being completely sure, you might not be able to fix damage that you caused later.

Be careful to not jump to conclusions too quickly without proper evidence(and not just what your child says alone). It wasn't all that long ago there was another fml where a very young child lied about her dad cheating on her mom and didn't admit it was lies until after they divorced(or were almost fully divorced, memories a little buggy).

Ooh, I really hope it's something innocent, like her friend's mom or teacher or grandmother. Please don't jump to conclusions!

"Sorry honey, your other mummy has died and I'm afraid Daddy's about to die from the same disease."

I used to say that to my mom too. About my whole family actually. My dad wasn't cheating or anything, I was just convinced I had another family before them.