How can one return the money he stole from his parents and siblings in the past, given that he does not remember the exact amount, and knowing that his father has passed away?

All praise is due to Allah, who guided the questioner to repentance. As for how to dispose of the stolen money, he should estimate all the money he stole until he is reasonably certain that he has encompassed it. Then he should return to each rightful owner what is due to them. He should return to his mother and siblings what he believes to be their share of the money. As for what he stole from his father, it is an inheritance that he should give to the heirs for them to divide among themselves. He is not obliged to reveal the full details to those to whom he returns the money. Rather, it suffices for him to say: "This is from someone who previously wronged you and has repented," or to the heirs: "This was money that your father was owed as a debt," and he has his share of the inheritance, which nothing can negate.