What is the ruling on the increase in my father's inherited savings account due to bank interest? Is it permissible to use this increase to pay off my brother's usurious debts or to cover arrears on my father's properties? And what is the ruling on inheritance funds in other bank accounts where it is difficult to ascertain their origins?

The excess money over the capital from usurious interest is forbidden usury. What is obligatory is to take it from the bank, while withdrawing the principal capital and donating the interest for the benefit of Muslims or giving it to the poor and needy. It is permissible to give it to a brother if he is unable to repay his debt. The questioner is not allowed to benefit from it unless he is among the poor. As for funds deposited in usurious banks where the original capital is unknown, it is obligatory to contact the banks to distinguish the principal from the interest. If this is not possible, then one should investigate and take out what achieves certainty or a strong presumption of absolution of liability.