I read in Saheeh Bukhari, in the chapter "Book of Marriage", that the scholars Az-Zuhri, Mujahid and some companions of the Prophet were of the opinion that if a man commits sexual intercourse with his mother in-law the marriage to his wife will not be invalid. Why is he not stoned to death as punishment as that is the Hadd punishment mentioned in many Ahadith regarding adultery by married man?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

No doubt that Zina is an obnoxious and bad sin.  So whoever commits it faces the wrath of Allah and His severe punishment.  This sin becomes filthier and more horrible when it is done with one of the Maharim (sister, mother, wife’s mother, ….etc.)

Indeed the Mahram man is required to protect the honour of his Maharim women and not to be the one who breaches it.

Anyway, the majority of scholars are of the opinion that committing Zina with the wife’s mother does not make the wife divorced and forbidden.  Ibn Hajar said in this respect, 'Ibrahim and Al Shaabi are of the opinion that if a man commits Zina with his mother in-law, both the wife and her mother become forbidden to him.  This is also the opinion of Abu Hanifa….'  But the majority of scholars think that this opinion is not correct because it is only the marriage to the woman that causes the prohibition of marriage to her daughter not Zina (forbidden intercourse).

The fact that this man deserves to be stoned does not influence the validity of his marriage to his wife just because he had sex with her mother.  Indeed the conditions of stoning are: 1) that the ruler be informed; 2) that 4 witnesses testify having seen this man commit the act.  If these two conditions are not met stoning cannot be applied and the wife remains in wedlock.  Even if stoning is applied she still is considered his wife.

Allah knows best.