Is the mother of the father’s wife a mahram to his sons?

Firstly: 

The father’s wife is a
mahram to his sons and his sons’ sons, because Allaah 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” [al-Nisa’ 4:22]. 

With regard to the mother
of the father’s wife, she is not a mahram to his sons, because there is
nothing to indicate that this is the case. Allaah, may He be exalted, says
after mentioning the women who are mahrams (interpretation of the meaning):
“All others are lawful…” [al-Nisa’ 4:24]. 

The scholars of the
Standing Committee for Issuing Fatwas were asked: My father has taken
another wife besides my mother. Is the mother of this wife a mahram to my
father’s sons from his other wife, or is she a non-mahram and is it haraam
for them to meet her, sit with her and be alone with her? 

They replied: The mother of
the father’s wife is regarded as a stranger (non-mahram) to his sons from
another wife and his sons are not mahrams to the mother of the father’s
wife. So she must observe hijab and it is not permissible for them to be
alone with her or for her to travel with them, because they are not mahrams
for her. End quote. 

Fataawa al-Lajnah al-Daa’imah, 17/358 

And Allaah knows best.