Step daughter

So I'm not sure if this is the right group... But I need some advice. My SO has a 7 year old daughter, who I've known since she was 3 when we got together. We did split up for awhile when she was 5. Lately she's been pretty disrespectful to me- giving me attitude, telling me and others she doesn't have to listen to me and I'm not an "adult" in her life. I love this girl like my own (without overstepping bounds) and I spend time with her, buy her clothes/food/treats. So I had a talk with her tonight about how it makes me kind of sad when she acts like that. And she finally admitted that her mother is telling her she doesn't have to listen to me at all because I'm "not her mother". And I get where her moms coming from, I have a daughter myself and if I thought another woman was trying to take my roll I'd be upset/jealous. I have no way of talking to her mom about it since I've never really met her- and no, not in a bad way, just usually pick up and drop off on the weekends is at her grandparents so that my SO and her mom don't wind up arguing. I explained to my step  daughter that I understood where her mom was coming from, and she did have to listen to her mom, just like she needs to listen to her dad when she's with us. But that I follow her dads rules for her and I am the other adult/stepparent and I would like her to be more respectful because I really care about her, but she's having trouble understanding how to follow all ours rules without making her mom upset and angry... I just don't know what to do :/ does anyone have any advice?  Sorry this is so long!