Is it permissible to pay the debt of the paternal uncle to the sister, and the debt of the father to the sister, from zakat funds, given that the debtor is unable to pay?

It is permissible for you and your sister to pay your zakat (definition) to your indebted uncle if he is unable to repay his debt, as he is considered among those in debt (al-gharimin) who are eligible to receive zakat. The permissibility of payment is further confirmed if he is also poor, in which case he is eligible for zakat from two perspectives. Paying zakat to him brings the reward of charity and strengthening kinship ties. He has the choice in how to dispose of the money; if he wishes, he may use it to pay off his debt or benefit from it in another way. It is not permissible to pay zakat directly to the creditor except with the debtor's permission.