Is the grandfather sinful for not helping his indebted son, knowing his need and his ability to pay off the debt by selling part of his property?

A financially capable father must provide for his needy child, even if the child is an adult and healthy. The same applies to the father’s grandchild, no matter how distant. Therefore, your financially capable grandfather is obligated to provide for his needy son and grandchildren. However, paying the father’s debts is not obligatory for the grandfather, nor is he required to compensate for past maintenance, as the maintenance of a relative lapses with the passage of time, unless ordered by a judge or incurred with his permission. It is preferable for the grandfather to voluntarily pay the debt if he is financially capable.