Can she uncover her face before her husband’s father through breastfeeding?
Question

                What is the ruling on a woman uncovering her face before her husband’s father through breastfeeding?.
Praise be to Allah.
    It is not permissible for a woman to uncover her face before 
    her husband’s father through breastfeeding according to the more correct 
    view which was favoured by Shaykh al-Islam Ibn Taymiyah, because the Prophet 
     (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 through blood ties for 
    his son’s wife, but he becomes a mahram of his son’s wife 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. End quote.

    Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on 
    him).