Is my girlfriend's stepmother a mother-in-law? Should I call mom?

In this case, you have to see what attitude your girlfriend is. If their families get along more harmoniously, or face to face, it is also considered your mother-in-law, you should also call auntie. No need to call yourself mom. Because after all, it is not your girlfriend's own mother, your girlfriend may not want, so this situation you need to discuss with your girlfriend.