Is the stepmother considered a forbidden degree of kinship (mahram) to her stepdaughter's husband?

The husband of a father's daughter is not a mahram to his wife's father's wife, because there is no reason for mahramiyyah (prohibition of marriage due to close relation) based on lineage, affinity, or suckling. Mahramiyyah applies to the wife's mother herself. It is not permissible for the father's wife to uncover herself in front of her husband's daughter's husband, nor to sit with him unless there is a mahram or another person present with them.