Are the heirs obligated to dispose of the bank interest that the deceased acquired before knowing it was unlawful, and then died without having disposed of it?

What your father used of the illicit gains, being ignorant of their unlawfulness and following those who deemed it permissible for him, bears no sin upon him. However, what remains with him, which he has not used, must not be left with the usurious bank, as that would be assisting it in its falsehood. It is obligatory to take it and dispose of it by spending it on the welfare of Muslims or giving it to the poor and needy. It is not permissible to benefit from it unless you are poor and in need of it, in which case you may take only what suffices your need. Unlawful money is not inherited; rather, it must be disposed of.