The baby mama drama

My husbands baby mama keeps trying to start crap. Her 5 year old calls me mom and i have corrected her many times telling her that her real mom is Stephanie and that I am her step mom. I will never tell this 5 year old not to call me mom if she feels like I’m a mother figure to her. Yet her biological mother feels extremely threatened by this. I’m not sure why if the 5 year old calls me mom when she’s with us it s. A big deal since Stephanie isn’t there to hear it. I can understand if she was around she may get hurt hearing her daughter refer to me as mom but when shes not it’s not a big deal to me because no one is hurt. She wants me and my husband to straighten the 5 year old out because she’s “confused” but I think telling her not to call me mom because Stephanie is her mom will make her more confused. She already understands that I’m her step mom and Stephanie is her real mother. What do you guys think?

The 5 year old told me today that she wants to call me mom and doesn’t want to stop. She even told me “I know Stephanie is my real mom and you’re my fake mom”. Could I tell her it’s ok she can call me mom and not tell Stephanie about it?