Is it permissible for a woman to lead a man in prayer, especially one of her mahrams?

A woman is not permitted to lead a man in prayer (imamah), whether he is a mahram (unmarriageable kin) to her or not. This is based on the hadith: "A people will not prosper who appoint a woman to lead them." It is also because a woman's place in prayer is behind men, as Umm Sulaym did when she prayed behind the Prophet, peace and blessings be upon him. Furthermore, the Imam is appointed to be followed, so how can one be led by someone for whom it is better to be behind?