Praise be to Allah.
The wife’s grandmother – whether through her father or her mother – is a mahram to her husband on a permanent basis as soon as the marriage contract is done with her granddaughter, because she is included in the verse in which Allah, may He be exalted, says (interpretation of the meaning): “Forbidden to you (for marriage) are:… your wives’ mothers” [an-Nisa’ 4:23]. So the mothers (and grandmothers) of wives are mahrams, no matter how far the line of ascent reaches.
Please see also the answer to question no. 161869
The sister of the wife’s grandmother comes under the same ruling as her maternal aunt, so she is prohibitted for marriage to the husband on a temporary basis, so long as his wife is still married to him. But if his wife dies or he divorces her and her ‘iddah ends, it becomes permissible for him to marry her grandmother’s sister. Similarly, the sister of the wife’s grandfather is also prohibitted to the husband on a temporary basis, because she comes under the same ruling as her paternal aunt.
The scholars of the Standing Committee said:
The grandfather is regarded as being the same as the father regardless of whether he is the grandfather on the father’s side or the mother’s side. End quote from Fataawa al-Lajnah ad-Daa’imah (17/355).
Al-Bukhaari (5109) and Muslim (1408) narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “One should not be married to a woman and her paternal aunt, or a woman and her maternal aunt, at the same time.”
See also the answer to question no. 175597
And Allah knows best.