Is Your Wife's Stepmother a Mahram?
Question

Is my wife’s stepmother my Mahram?
Praise be to Allah.Is Your Wife’s Stepmother a Mahram?
The wife of your wife’s father is not considered to be a Mahram for you, so it would be permissible for you to marry her, because whether or not a person is a Mahram can only be proven by a text of Shari`ah, and there is no text to state that this person is a Mahram. Rather, when Allah listed the women who are Mahrams , He stated that all others are permissible for marriage. Allah says (interpretation of the meaning):
{All others are lawful.} [An-Nisa’ 4:24]
Not only that, it is also permissible to be married to a woman and her father’s ex-wife at the same time, according to the majority of scholars.
Scholarly Opinions on Marrying a Wife’s Stepmother
Ibn Rajab Al-Hanbali (may Allah have mercy on him) said:
Being married to a man’s ex-wife and his daughter from another wife at the same time is permissible according to the majority but is disliked according to some of the Salaf. (End quote from Jami` Al-`Ulum wal-Hikam, p. 411)
Imam Ash-Shafi`i (may Allah have mercy on him) said:
If a man is married to the daughter of a man and the ex-wife of her father (at the same time), Abu Hanifah (may Allah have mercy on him) said that this is permissible, and we heard that `Abdullah ibn Ja`far did that.
Ash-Shafi`i (may Allah have mercy on him) said: There is nothing wrong with being married to a man’s ex-wife and his daughter from another wife. (End quote from Al-Umm, 7/155)
Imam Ibn Hazm (may Allah have mercy on him) said:
It is permissible for a man to be married to a woman and to the ex-wife of her father and the ex-wife of her son and the daughter of her paternal uncle at the same time, because there is no text which states that this is haram. This is the view of Abu Hanifah, Malik, Ash-Shafi`i and Abu Sulayman. (End quote from Al-Muhalla, 9/532)
Ibn Qudamah (may Allah have mercy on him) said:
There is nothing wrong with being married to a woman who was the wife of a man and his daughter from another wife.
Most of the scholars say that it is permissible to be married to a woman and her stepdaughter at the same time. `Abdullah ibn Ja`far and Safwan ibn Umayyah did that. This is the view of all the jurists apart from Al-Hasan, `Ikrimah and Ibn Abu Layla; it was narrated that they regarded it as disliked.
Allah says (interpretation of the meaning): {All others are lawful.} [An-Nisa’ 4:24]
And because they are not related, they are like two strangers. And because marrying two closely-related women at the same time is forbidden lest there be a severing of family ties between those who are closely related. There is no such relationship between these two, so their case is different from what has been mentioned. (End quote from Al-Mughni, 7/98)
Based on this, then the wife of your wife’s father is not counted as one of your Mahrams, rather she is a “stranger” to you, so you may not shake hands with her or be alone with her or travel with her.
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And Allah knows best.