It's my mom trying to get me to upset my stepmom?

I'm 15. I have been living with my dad and stepmom since I was 5. My mom has been in and out of my life. My mom bought me a new iPhone and she put it on her plan. My Mom told me I can have it but my stepmom is not allowed to touch it and she is not allowed to through my call logs or text messages. My mom said I'm you mother not her, your my my daughter not hers and she doesn't have a right to tell you what you can and can't do. This is annoying me because my mom has always these kind of conditions. I don't want the stupid iPhone because it's going cause problems. It's like she always trying to get me to upset my stepmom.

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