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

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).