What should a wife do if her husband's brothers see her, even though she is covering her face and head?

A woman must safeguard herself from being seen by non-mahram men, including her husband's brothers. If one of them sees her unintentionally on her part, there is no sin upon her, based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful." It is the wife's right that her husband provide her with an independent dwelling where she feels safe from being seen by non-mahram men.