Is it permissible for the veiled wife of a brother to uncover her face in front of her husband's unmarried brother, and is smelling her perfume in the house considered fornication?

If your brother's wife considers the niqab obligatory, then it is not permissible for her to uncover her face in front of non-mahram men. This is particularly emphasized with the husband's relatives, such as his brothers, due to the Prophet's (peace be upon him) saying: "The in-law is death." As for using perfumes and fragrances inside the house, there is no sin in it, as long as she does not intend for non-mahram men to smell her fragrance. This is because the prohibition is specific to going out or intending for a non-mahram man to be perfumed by it, due to his (peace be upon him) saying: "Any woman who perfumes herself and then goes out and passes by a group of people so that they may smell her fragrance, she is an adulteress," and his saying: "Any woman who uses incense should not attend the last 'Isha' prayer with us."