A General Rule of Who a Mahram is
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Mahram refers to person with whom nikaah is haraam, either due to blood relationship, or marital, or milk-ties.
Further requisites for a mahram [for going along with in Haj] is that he is trustworthy, of sane mind and of majority age in the Shariah.
(Mukammal Wa Mudallal Masaail-e-Haj Baraae Khawaateen)