A woman cannot be a mahram for another. The one who is considered mahram is a man that a woman
cannot marry due to blood relations, such as her father and her brother, or a man related to her due to marriage, such as
her husband, her father-in-law and her step-son, or a man related due to breast feeding, such as her father from breast
feeding and so forth.
It is not allowed for a man to be in private with a woman he is not related to nor can he travel with her. The Prophet
(peace be upon him) said,
"A woman does not travel except with a mahram."
This was recorded by al-Bukhari and Muslim. The Prophet (peace be upon him)
also said,
"A man is never alone with a woman except that Satan is the third."
This was recorded by Imam Ahmad and others from the hadith of Umar with a sahih chain.
