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

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.