Your father’s wife’s mother is not a mahram for you
Question

                My father took a second wife and it is known that she has become a mahram for me, but my question is: Has her mother also become a mahram for me?.
Praise be to Allah.The mahrams through 
    marriage fall into four categories: 

    1.The ascendants of the husband 
    [his father and grandfathers] become mahrams to the wife.

    2.The descendents of the husband 
    [his sons and grandsons] become mahrams to the wife.

    3.The ascendants of the wife [her 
    mother and grandmothers] become mahrams to the husband.
These three categories 
    become mahrams as soon as the marriage contract is done.

    4.The descendants of the wife 
    [her daughters and granddaughters] become mahrams to the husband. In this 
    case there must be consummation. Once consummation takes place, then her 
    daughters from a previous or subsequent husband become mahrams to him 
    forever.
End quote from al-Sharh 
    al-Mumti’ (12/128). 
Thus it is clear that the 
    mother of the father’s wife does not become a mahram for his sons. 
It is permissible for a man 
    to marry a woman, and for his son to marry her mother, or her daughter, 
    because that is included in the words of Allaah, after mentioning the women 
    who are mahrams (interpretation of the meaning): 
“All others are lawful”
[al-Nisa’ 4:24]
And Allaah knows best.