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

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.