Is a person sinful for keeping lawful money that has been mixed with usurious gains from his father, knowing that he cannot dispose of it until after his father's death, and he does not intend to use it before getting rid of the usurious gains?

If your father gives you lawful money mixed with interest-based earnings as a completed gift, there is no harm in you benefiting from it, especially if the lawful money is predominant. You should gently and wisely advise your father to purify his wealth, rid himself of unlawful earnings, and repent to Allah.