I have one suggestion...when you hear her tell anyone that you are not her mom, why don't you walk up to that person (whether it's a child or adult) and extend your hand for a hand shake and say, "Hi, my name is so-in-so, I'm so-in-so's stepmom. What is your name?" This is what I do everytime. It let's every person know who you are personally, one on one. They will get to know you and announce your name or so-in-so's stepmom when they see you.
I do this with everyone at school, sports, supermarket, anywhere that my stepdaughter runs into someone she knows. It also let's word get back to mom that you are representing yourself as stepmom just in case she had any concerns otherwise.
I've had that problem many times with people. Sometimes my stepchildren clarify it with, "she's my stepmom" and sometimes they don't even correct people because it's not that important especially when it's people they may never see again (cashier, etc).
I don't like the, "I married your dad thing." It doesn't sound personal enough for me. It's makes me hear, "I married your dad and now you're stuck with me."
In fact, their father and I would not have gotten married at the time that we did if the children had a problem with it. He actually took each one separatly into a room and asked their permission to marry me. It really made them feel important to our relationship. We explained that it's not just their father and I that are getting married but it's our two familys (me & my son, him & his two children) that are getting married.
Guess I'm just different that way.
I had a problem with my stepdaughter only once when she was 5. I disciplined her by making her sit in the corner (Dad's method of punishment) for being mean to her brother. She knew she had to be quiet for 5 minutes before she could get up. She threw a fit and screamed at me. I ignored her. She then yelled, "You're mean like my mom said, you are not my mom and my mom said I don't have to listen to you!" I just replied, "You will only have one mom. I am your stepmom. You will listen to me when you are with me because I am in charge." I then ignored her.
When dad got home, I let him know what happened and he took her to a room and had a long talk with her.
We never had a problem like that out of her again. Of course mom still tries to turn them against me, but they have learned to ignore most of her manipulations.