What is the ruling on a son refusing to give money to his father to cover his sister's expenses, knowing that the father is well-off and his retirement pension is twice the son's salary, and the son is heavily indebted?

As long as you are in need of your money to pay off your debt, and your father is not poor, then you are not obligated to give him your money, and he has no right to supplicate against you. However, you must honor him and treat him kindly in ways that you can and that do not harm you. If you honor him, his unjust supplication against you will not harm you, Allah willing.