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

                Is the mother of one’s mother’s husband regarded as a mahram of the wife’s sons?.
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 there 
    is no relationship of mahram between the ascendants and descendants of the 
    husband, and the ascendants and descendants of the wife. The husband’s 
    mother is not a mahram to his wife’s sons, so it is not permissible for her 
    to shake hands with them or be alone with them or travel with them. 
And Allaah knows best.