Is the mother of the stepfather considered forbidden to the children of the wife?

Prohibited degrees of marriage by affinity are of four types: the ascendants and descendants of the husband vis-à-vis the wife, and the ascendants of the wife vis-à-vis the husband. These three types become prohibited immediately upon the marriage contract. As for the descendants of the wife vis-à-vis the husband, they become prohibited on condition of consummation. Accordingly, there is no prohibition between the ascendants and descendants of the husband and the ascendants and descendants of the wife, which means that the husband's mother is not prohibited for his wife's sons.