My future step daughter doesn't understand.

My soon-to-be step-daughter (B) is 5 and doesn't really understand what a wedding is, obviously because she is 5. We have never referred to me as mom or anything close to it because I don't want to take that away from her mother, who hates me. I want B to be involved as much as possible because she is already so excited when she is helping me with little things, but how do we explain it to her simply and clearly so when people start referring to me as step-mom she isn't confused??? I am so lost and I don't want to cause any more drama than we already have to with bio-mom. There has been so many times that she almost calls me mom because that's how she sees me, as a mother figure, but she pauses and isn't sure what to call me so it goes back to "friend" or "Kami". Please if any one has explained this before to their kids or step kids I would really appreciate some advice.