The Function of a Mahram
A mahram is a close relative with whom marriage is forbidden for all time. Marriage can never at any stage become lawful with such a person. The normal hijaab restrictions are not applicable for a mahram.
Father, sons, grandfathers, true uncles (maternal and paternal), nephews (whether from the brother’s or sister’s side) and grandsons are among the mahareem (plural of mahram) of a woman.
On a Journey
When a woman goes on a journey, it is compulsory that she be accompanied by a mahram. Rasulullah (Sallallahu alayhi wasallam) said that it is not lawful for a woman who believes in Allah and the Last Day to go on a journey without a mahram.
While the essential requisite for a woman who intends to travel is a mahram to accompany her, there are some necessary conditions which qualify a mahram to accompany the woman on her journey. The primary function of the mahram is to safeguard the hijaab and honour of the woman. He has to incumbently see to all her requirements along the journey. He should endeavour to the best of his ability to ensure that violation of her hijaab does not occur. He has to guard her from any fisq and fujoor.
If the mahram is a careless person who is unconcerned, permitting unnecessary violation of Purdah rules, then he is not a mahram who qualifies to be the guardian of a woman on a journey. Such a person is described as a faasiq by the Shariah. It is not permissible for a woman to travel in the company of a faasiq even if he happens to be her father, son or brother.
When a mahram fails in the execution of his primary function, he is disqualified by the Shariah, and the woman may not travel with him. This applies to all journeys, not only the Hajj journey.
(The Majlis Archives)