How can a father be convinced to pay his old debts, some of whose owners have passed away, given existing family disputes about this matter? And is it obligatory to give him the entire salary, knowing that he spends money quickly?

Your father must repay his debt if he is able. The creditor's right is not forfeited by the passage of time. However, if he is unable to do so, then neither he nor his children are obligated to repay it; rather, their only obligation is to provide for his sustenance. It is an act of filial piety to strive to repay your father's debt by asking him about his creditors and encouraging your siblings to cooperate in settling it. Then, restore the rights to their owners or their heirs. If your siblings are unable or do not participate, your repayment of the debt will clear your father's conscience, and you will receive a great reward. The gravity of debt in the Afterlife is indicated by its significance. The Prophet, peace and blessings be upon him, said: "The soul of a believer remains suspended by his debt until it is paid off," and "Every sin of a martyr is forgiven except debt." The matter should be pursued with wisdom and gentleness to avoid estrangement.