Is the stepfather considered a mahram to the girls, and is the brother's wife considered a mahram, and must the brother's wife observe hijab in front of him?

A mother's husband is considered a mahram (unmarriageable kin) to her daughters. If a man has consummated his marriage with the mother, her daughters become unlawful for him (to marry), based on the Almighty's saying: "And your stepdaughters who are in your guardianship, [born] of your women unto whom you have gone in. But if you have not gone in unto them, then there is no sin upon you." As for the mother's husband, he is not a mahram to his son's wife, and it is not permissible for him to be alone with her or to look at her.