Is the husband of my grandmother from father’s side a Mahram (non-marriageable relative) for me and for the sister of my father?

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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The grandmother and great-grandmother is like the mother, and the husband of the grandmother is like the husband of the mother. So the grandmother of your father is your grandmother, and her husband is like the husband of your mother, i.e. he is a Mahram (non-marriageable relative) for you. He is also a Mahram for your father’s sister if the grandmother of your father is also her grandmother.