Is it permissible for a wife to expose her face before her husband's maternal grandfather?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

It is permissible for a wife to expose her face before her husband's paternal or maternal grandfather. Allaah Almighty says (what means): {And not display their beauty except to their husbands, their fathers, their husband's fathers...} [Quran 24: 31]

The husband's fathers include all grandfathers from all sides. 

Allaah Knows best.