Is the wife of one’s stepson (wife’s son from a previous marriage) one of the mahrams of his mother’s husband?
Question

                Is the wife of my wife's son one of my mahrams?
Praise be to Allah.
    The wife of a stepson is not one 
    of the mahrams of his mother’s husband, because the factors that make a 
    person a mahram are three: blood ties, breastfeeding and marriage (in-laws). 
    The wife of a stepson does not come under any of these three headings. 
    Allah, may He be exalted, has mentioned the women who are mahrams in the 
    verses in which He says (interpretation of the meaning):

    “And marry not women whom your 
    fathers married, except what has already passed; indeed it was shameful and 
    most hateful, and an evil way.

    23. Forbidden to you (for 
    marriage) are: your mothers, your daughters, your sisters, your father’s 
    sisters, your mother’s sisters, your brother’s daughters, your sister’s 
    daughters, your foster mothers who gave you suck, your foster milk suckling 
    sisters, your wives’ mothers, your stepdaughters under your guardianship, 
    born of your wives to whom you have gone in — but there is no sin on you if 
    you have not gone in them (to marry their daughters), — the wives of your 
    sons who (spring) from your own loins, and two sisters in wedlock at the 
    same time, except for what has already passed; verily, Allaah is 
    Oft‑Forgiving, Most Merciful.”

    [an-Nisa’ 4:22-23]. 

    Shaykh as-Sa ‘di (may Allah have 
    mercy on him) said: all of those who are not mentioned in this verse; they 
    are permissible and good. Thus what is prohibited is limited, but there is 
    no limit to or restriction on that which is permitted. This is the kindness 
    and mercy of Allah, and is intended to make things easy for people.

    End quote from Tafseer as-Sa‘di 
    (p. 174) 

    See also al-Muhalla by 
    Ibn Hazm (9/532) 

    The scholars of the Standing 
    Committee (17/372) were asked: 

    My wife wears niqab in front of 
    my mother’s husband. Can he see her face? Please note that he is the one who 
    brought me up from a young age. 

    They replied: The mother’s 
    husband is not a mahram for the wife of his stepson, so she must observe 
    hijab with him, because he is a non-mahram to her.

    End quote. 

    And Allah knows best.