What is the ruling on the money that a brother took from his father to pay off his debts, and did not return, and is he entitled to take his lawful share of the inheritance? And is the father sinful for not treating his children equally?

The answer can be summarized in two points:

First: The money the father paid to settle his son's debt: - If the father paid it as a loan and did not waive it, then the son must repay it to the heirs after the father's death. This does not prevent him from inheriting. If he refuses, the heirs may take him to court. - If the father paid it as a gift, then there is no debt on the son, and there is no harm in this preference if the son was in need. As Shaykh al-Islam Ibn Taymiyyah mentioned, one of the justifications for preference is settling a debt.

Second: We advise the son to fear Allah and repay the money if it was a debt and he is able to, or return it if it was a preference without a legitimate justification, in order to clear his father's conscience and preserve family cohesion.