Is my wife's paternal grandmother a forbidden relative (mahram)?

The wife's grandmother (from her father's or mother's side) becomes unlawful for the husband upon the mere marriage contract, as she falls under the verse: (وَأُمَّهَاتُ نِسَائِكُمْ) "and your wives' mothers." The mothers of wives are forbidden, no matter how high up the lineage they go, and scholars have unanimously agreed upon this. Consequently, the wife's grandmother is a mahram, and it is permissible for her to uncover her face in front of him and to be in seclusion with him.