You need to set up boundaries. When my grandmother got remarried after her divorce, her second husband tried to get us to call him "grandpa". Now, looking back he didn't have any family except for nieces he never saw, so he probably just wanted the feeling of family. But we just called him by his first name because our grandfather was still alive and he was our grandfather, not her new husband.
Instead of calling her "grandma", just call her by her first name or whatever name you feel is appropriate. If you are asked why, simply say that you have a grandmother and this woman is not her. That you respect her position in the family and her relationship with your grandfather, but she is not your grandmother.
It may also be time to start talking to your grandfather about how she is making you feel. I don't think he would like it if his grandchildren were being made to feel excluded or anything. It sounds like he got remarried so quickly because he just doesn't know how to live by himself, which is understandable. But you are fair to be wary about this new woman.
Make sure your parents/aunt and uncles keep an eye on his health and finances. If it seems like she is trying to isolate him or he suddenly runs out of money or starts going on expensive trips that are her idea, be careful. She could be trying to use him for his money.