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.