Is the maternal uncle a forbidden marriage for the questioner’s sister? And are there cases in which a woman is permitted to uncover her face in front of non-mahrams, such as the impotent man, and what are these cases?

Your mother's maternal uncle (khal) is, by consensus, a mahram to your sister from your mother, as Allah the Almighty says: "Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother's daughters, and your sister's daughters." It is permissible for a woman to uncover her face and adornments when necessary, such as for medical treatment with a doctor, and in front of those who have no desire for women, such as children and eunuchs.