All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is permissible for a man to be married to a woman and her stepmother at the same time. It is narrated that ‘Abdullaah ibn Ja‘far was married to both the daughter of ‘Ali both, and a woman who used to be Ali’s wife at the same time. [Al-Bukhari]
Ibn Seereen did not object to a man’s marriage to a woman and her step-mother. [Al-Bukhari]
There is no evidence indicating that such a marriage is disliked under Sharee’ah. The preponderant opinion is that it is permissible.
Allaah Knows best.