Is it obligatory for someone who stole a large sum of money from his deceased father, and has brothers, sisters, and a mother, to compensate them for the money he stole, despite his difficult financial circumstances?

The right must be paid to the heirs and distributed according to the legal shares. If the money is not available, forgiveness can be sought from the adult heirs. If they do not agree, it is obligatory to pay them, even in installments, with the intention to pay whenever it becomes possible.