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

                Is the mother of the father’s wife regarded as a mahram to the husband’s sons?.
Praise be to Allah.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.