Praise be to Allah.
It is not permissible for a woman to uncover her face in front of her husband’s father through breastfeeding, according to the correct view which was favoured by Shaykh al-Islam Ibn Taymiyah, because the Messenger (peace and blessings of Allaah be upon him) said: “The same people become mahrams through breastfeeding as those who are mahrams through blood ties.” The husband’s father is not a mahram for his son’s wife through blood ties, but he becomes a mahram through the marriage tie, because Allaah says (interpretation of the meaning):
“Forbidden to you (for marriage) are: … the wives of your sons who (spring) from your own loins”
A son through breastfeeding is not the same as a son from one’s own loins. On this basis, if a woman’s husband has a father through breastfeeding, then she must observe hijaab in front of him and not uncover her face in front of him. If we assume that she were to separate from his son through breastfeeding, it would not be permissible for her to marry him, in order to be on the safe side, because this is the view of the majority of scholars.