Is it permissible for one to give zakat to one's widowed or divorced sister?

If a widow or a divorcee does not have working adult sons to support her, it is the reponsibility of her brothers to support her. This responsibility of providing for one's dependents in nafkah (by bloodline or marriage) cannot come in the form of giving zakat because zakat is for those who are not one's dependents who are under one's responsibility to support.