Is my wife's paternal grandmother considered one of my Mahrams?

Your wife's paternal stepmother is not considered among your mahram (unmarriageable kin), and it is permissible for you to marry her, as there is no textual prohibition against it. Allah has permitted what is not explicitly forbidden. It is permissible to marry a woman and her paternal stepmother concurrently, according to the majority of scholars, because a prohibition must be based on a religious text, and no text has been revealed to prohibit this. Rather, the default is permissibility, based on the Almighty's saying: "And lawful for you are [all others] besides these."