Dr. Marawan Shahin, Professor of Hadith and its Sciences, Faculty of Usul Ad-Din (Theology),Al-Azhar Univ. states: “A man can be a mahram for his mother-in-law while going for Hajj. He is a mahram to her as he cannot marry her. May Allah reward all pilgrims and grant them Hajj mabrur.”
Allah Almighty knows best.