Is the mother's husband considered a mahram to the woman, and what is the evidence for that?

Affinity is one of the causes for the prohibition of marriage. The noble verse (An-Nisa: 23) mentions the stepdaughter (the daughter of a man's wife from another man) among those forbidden for marriage. The stepdaughter is forbidden to her stepfather if he has consummated the marriage with her mother, whether she was raised in his household or not, based on the Almighty's saying: "And your stepdaughters who are under your guardianship [born] of your wives with whom you have consummated marriage." However, if he contracts marriage with her mother and then divorces her or she dies before consummating the marriage with her, then the stepdaughter is a stranger to him, and it is permissible for him to marry her, based on the Almighty's saying: "But if you have not consummated marriage with them, then there is no blame upon you."