What should a wife do if she is given the choice between divorce or abandoning the niqab, which she considers to be an innate disposition, especially if the husband says it is not obligatory? And is it permissible for her to remove it in front of his brothers based on his statement that it is a Sunnah?

A woman must cover her face in front of non-mahram men, and this includes her husband's relatives. In fact, she must observe a more stringent hijab in their presence, as they have more frequent interactions with her. It is not permissible for the husband to order his wife to uncover her face, as this is an act of disobedience (to Allah), and there is no obedience to a creature in disobedience to the Creator. However, if he threatens her with divorce, it is permissible for her to uncover her face and hands, based on the saying of the Prophet (peace be upon him): "Indeed, Allah has forgiven my Ummah for mistakes, forgetfulness, and what they are forced to do." Necessity is determined by its extent; if the harm is removed by her uncovering only in front of his brothers, then it is not permissible for her to uncover in front of others.