Is it permissible to give zakat to non-Muslim relatives, considering them to be among those whose hearts are to be reconciled (Al-Mu'allafati Quloobuhum)?

It is not permissible to give zakat to a disbeliever (kafir) except in the case of those whose hearts are to be reconciled (al-mu'allafati qulubuhum). If your father gave zakat to his disbelieving relatives without any indications of their inclination towards Islam, then the zakat is not valid and must be repaid. However, if there were indications of their inclination towards Islam, then there is a difference of opinion among scholars, and the apparent view is that it is valid. Nevertheless, it is preferable to avoid giving them zakat in the future to steer clear of this difference of opinion.