Uncovering in front of the husband’s father through radaa’ah
Question

                What is the ruling on a woman uncovering her face in front of her husband’s father through radaa’ah (breastfeeding) [i.e., the husband of the woman who breastfed him in infancy]?
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”

    [al-Nisa’ 4:23]

     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.