Is my wife’s paternal grandmother a mahram, i.e., can she uncover her face in front of me?.
The wife’s grandmother – either paternal or maternal – becomes a mahram to the husband once the marriage contract is done, because she is included in the verse in which Allah says (interpretation of the meaning):
“your wives’ mothers”
Thus the mothers, grandmothers and great-grandmothers of the wife, no matter how far the line of ascent reaches, are mahrams to the husband.
It says in Zaad al-Mustaqni‘: His wife’s mother and her grandmothers become mahrams once the marriage contract is done.
See al-Sharh al-Mumti‘, 12/118
It says in al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah, 15/121:
The fuqaha’ are unanimously agreed that it is haraam to marry grandmothers in all cases, whether they are through the mother or through the father, no matter how far the line of ascent reaches, and that is because Allah says (interpretation of the meaning): “Forbidden to you (for marriage) are: your mothers” [al-Nisa’ 4:23]. Mothers are all those to whom one is attributed through birth, whether they are called mothers in the real sense or metaphorically – i.e., the one who gave birth to you or the one who gave birth to the one who gave birth to you – no matter how far the line of ascent reaches, and whether they are heirs or not. End quote.
Based on that: she may uncover her face in front of you.
And Allah knows best.