The husband of a half sister from the father is not a Mahram
Is it permissible for my mother to unveil her face in the presence of my paternal half sister's husband?
It is not permissible for your mother to uncover her face in the presence of the husband of your paternal half sister, for he is not of her Mahrams (unmarriageable relatives). May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions.