I don't want to call my dads step-mum grandma
Why don't you want to call her grandma? Did something happen between you two?
Yes, when I was a kid, she once refused to give me candies when she knew I would be asking for it. She's mean.
That seems like an overreaction to that particular incident, but it's perfectly fine to not want to call her grandma. It's your choice, and no one should force you to say something that you don't feel comfortable saying.
Good. I'm glad you understand.
I understand where you're coming from. It's perfectly reasonable for you to not call her grandma.