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

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.