Is it permissible to give zakat to a married brother who sometimes asks for money to cover his household expenses, and to another unemployed brother who lives with his retired mother? And is it obligatory to pay zakat again if it becomes clear that it was not permissible to give it to them in previous years?

It is permissible to give zakat to a brother under two conditions: that he is deserving of it due to poverty or debt, and that the muzakki (person giving zakat) is not obligated to provide for him (i.e., would not inherit from him if he died). Based on this, it is permissible to give zakat to the first brother if he is poor and has a male child who would prevent the muzakki from inheriting. As for the second brother, if his upkeep is obligatory upon the mother and her income is insufficient, then it is permissible to give him zakat because his upkeep is not obligatory upon the muzakki. Sheikh Ibn Uthaymeen clarified that giving zakat to a brother is permissible if it does not relieve the muzakki of an obligation.