Praise be to Allaah.
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):
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‘,
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.