The husband’s grandfather is one of his wife’s mahrams

Praise be to Allah.

May Allah reward you with good for your concern about religion and your keenness to adhere to that which Allah has made obligatory upon you. With regard to your question about the husband’s grandfather, there is no doubt that he is a mahram. The husband’s grandfather, whether he is the father of his father or the father of his mother, is a mahram. So there is no blame on you for uncovering your face and hands and so on in front of him, and there is nothing wrong with that. He is like all other mahrams such as your brother, uncles and so on. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).