Is it permissible for a brother's wife to uncover her face in front of the younger brother whom her sister raised?

If your father's wife did not breastfeed your younger brother, then she is not his mother, and her sister is not a mahram to him. His status with her sister is the same as that of his older brother who married her. Thus, it is not permissible for her to uncover before him what she uncovers before her mahrams. Your father's wife herself is a mahram to your brother, but her sister is not considered a mahram to him merely by virtue of upbringing that did not involve breastfeeding.