Need help with my step child

She a great little girl. Ive been in her life since she was months old. She loves me and respects me. She listens very well to her dad and i. My problem is she isnt listening to her mom very much. We have about 50/50 custody. She is in school and has to wake up early to be dropped off at morning daycare so everyone can get to work on time. With me she wakes up no problem and gets ready and i do her hair. Shes done in 15 min. She isnt as easy for her mom and her mom has her younger sister to get ready as well.

Last night my step daughter willingly told me that she got to sleep in because she was being bad. I said that doesnt make sense sleeping in isnt a punishment. She said i know and then told me her mom told her to wake and she didnt because she didnt feel like it. I told her she cant do that and that she needs to do as she is told because her mom has to get ready as well for work and she is making things harder by not listening. As well as that her mom is the adult in charge and she needs to listen to whatever adult is in charge of her at anytime. She then said well kids dont have to listen. I said your right but then you get in trouble for not doing so and is that fun. She said no. I told her that if she gives her mom problems that her dad and i can take away stuff in our home from her for not being good. She said i know i only listen when my mom says she is going to call my dad. I had asked why she does that and she didnt really have an answer. My SO had walked in on the conversation at that point and told her if her mom has to call him in the morning and he has to go over there and get her out of bed for school he will be mad and he will take all of her toys away from her. This isnt new behavior as she has been doing stuff like that for years with her mom and her mom always ends up having to use my SO as a way to get her to listen. It just recently started with the waking up as she started kindergarten this past year.

This is the first time i really ever had my step daughter willingly tell me she was being bad and that she doesnt want to listen to her mom when she is told to wake up. To say the least this was a very insightful conversation as she came to me and talked about it to me so i got more of her way of thinking out of the conversation. Im not sure how to fix this problem. Im going to talk to her mom about it so she knows about this conversation i had with daughter. I just honestly dont understand as to why daughter wont listen to her mom the way she does her dad and i. We have taken thi gs away from her in our home for not listening to her mom. Also her moms punishments differ from my SO. Her mom does spank we dont and have never had to because daughter listens to us and my SO is against spanking but he does say he will do it just never has or has had to do it. Is there a way to get her to listen to her mom