Is my wife's son's wife considered a mahram?

The wife of a stepson is not a mahram to her husband's stepfather, because there is no reason for mahramiyyah based on lineage, breastfeeding, or marriage. The Noble Qur’anic verse mentions the prohibited categories, and everything not mentioned therein is permissible. The Permanent Committee has issued a fatwa stating that a stepfather is not a mahram to his stepson’s wife.