Is the husband of my father’s grandmother a Mahram to me and to my father’s sister?

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.

A grandmother of all degrees is like a mother. Also, the husband of a grandmother is like a mother’s husband. So, the grandmother of your father is your grandmother and her husband is like the husband of your mother, and thus he is a Mahram to you. He is also a Mahram to your father's sister if your father’s grandmother is her grandmother as well.