Is a man considered a mahram to his brother's wife for the purpose of 'Umrah and Hajj?

The husband's brother is not a mahram to his brother's wife. Therefore, it is not permissible for a woman to rely on him as a mahram for travel, whether for Hajj or other purposes. This is indicated by the Prophet Muhammad's (peace be upon him) saying concerning the husband's brother (al-hamw): "The hamw is death." As for the Hajj, it is valid, Allah willing, if its pillars and obligations have been fulfilled.