What do you call a step grandma?

So I'm running into a major dilemma with what to call my step-mom. My dad had an affair with her and left my mom after 25 years of marriage, so it's not like she held an important role of raising me at all. But, my baby will need to call her something. Because my mom and her have the same last name, it will be confusing if they're still Grandma _______. Should we think of another name to call her or call her by her first name (Grandma Jamie) or what? My mom would be livid if she wasn't Grandma ______. Suggestions? Too much drama for me!