What is the ruling regarding someone who had sexual intercourse with his father's wife, and does the father's marriage become null and void, and what is the reason for that, and does the ruling differ from if he had sexual intercourse with his own mother?

Adultery is a major sin, and its ugliness and criminality increase if it is committed with a mahram (definition) relative, such as one's father's wife. Adultery with a mahram is more forbidden than adultery with a non-mahram. Scholars have differed regarding the punishment for one who has intercourse with his father's wife. Some said he should be killed absolutely, which is the opinion of Imam Ahmad and was preferred by Ibn Al-Qayyim. Others said his punishment is the same as that of the adulterer (stoning for the married person and flogging for the unmarried person), which is the opinion of Al-Hasan (definition), Malik, Al-Shafi'i, and Abu Hanifa. The evidence points to the opinion of killing, such as the hadith (definition) of Al-Bara' ibn 'Azib, in which the Prophet (peace and blessings be upon him) sent someone to kill a man who married his father's wife and seize his property, and the hadith of Ibn 'Abbas: "Whoever has intercourse with a mahram relative, kill him."

Regarding the question "Does the father's marriage become annulled...", scholars have differed on this matter. Some said it is annulled, while others said it is not, which is the opinion of Al-Shafi'i. As for intercourse with one's mother, it is more severe, uglier, and graver, and its perpetrator should be killed as a matter of priority, and his crime is greater than the crime of one who committed adultery with his father's wife.