Summary of answer:
Praise be to Allah
The Messenger (blessings and peace of Allah be upon him) forbade women to travel without a mahram. He said: “No woman should travel unless she has a mahram with her, and no man should enter upon her unless she has a mahram with her.” A man said: O Messenger of Allah, I want to go out with such-and-such a campaign, and my wife wants to go for Hajj. He said: “Go for Hajj with her.”
Narrated by al-Bukhaari (1862) and Muslim (1341).
Who is a woman’s mahram?
A woman’s mahrams, with whom it is permissible for her to travel, are: her husband, or anyone who is permanently forbidden to marry her due to ties of blood or some other valid reason.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
Her mahram is her husband, i.e., the one who has done a valid marriage contract with her, even if no intercourse has taken place and he has not been alone with her.
The words “or anyone who is permanently forbidden to marry her” exclude anyone for whom she is temporarily prohibited, such as a woman who is in ihram for Hajj or ‘umrah.
“due to ties of blood” means due to kinship
“or some other valid reason”. Valid reasons may be divided into two categories:
2. Ties through marriage
With regard to ties through blood or kinship, the mahrams in this category are: the father, son, brother, paternal uncle, brother’s son, sister’s son, and maternal uncle. These seven are mahrams due to ties of blood, and it is permanently forbidden for them to marry the woman.
Mahrams through breastfeeding are the same as the mahrams through blood ties. Those who are a woman’s mahrams through breastfeeding are: her father through breastfeeding, her son through breastfeeding, her brother through breastfeeding, her paternal uncle through breastfeeding, her maternal uncle through breastfeeding, the son of her brother through breastfeeding, and the son of her sister through breastfeeding. So there are seven through breastfeeding and seven through ties of blood. That is fourteen.
Mahrams through ties through marriage are four: the father of the woman’s husband, the son of the woman’s husband, the husband of the woman’s mother, and the husband of the woman’s daughter. They are the ascendants of her husband, namely his father and grandfathers, and his descendants, namely his sons and the sons of his sons and daughters, no matter how far the line of descent reaches. These mahrams also include the husband of her mother and the husband of her daughter. But there are three who become mahrams as soon as the marriage contract is done, namely the father of the woman’s husband, the son of the woman’s husband and the husband of the woman’s daughter. As for the husband of her mother, he does not become a mahram unless he has consummated the marriage with her mother.
End quote from ash-Sharh al-Mumti‘ (7/37-38)
For more information, please see fatwa no. 5538
If this mahram – such as her father or brother – is a disbeliever, can she travel with him and be alone with him when she is a Muslim?
There are two scenarios:
The first scenario is when this mahram who is a disbeliever has dubious morals or follows some evil ideas or religion that permits intimacy with mahrams. In that case, it is not permissible for a Muslim woman to travel with him or be alone with him, because the fear of fitnah from him is like the fear of a stranger, or even worse.
Ibn Qudaamah (may Allah have mercy on him) said:
It is not appropriate for there to be any difference of opinion concerning a Magian, because he is not to be trusted with her and he thinks that she is permissible for him.
End quote from al-Mughni (5/34)
We may add to the category of the Magian anyone who follows a similar evil path of believing that it is permissible to be intimate with mahrams.
The second scenario is when he is trustworthy. Most of the scholars were of the view – and this is the correct view – that he is a mahram for her with regard to travel, so she may travel with him, because he is to be trusted with her in this case.
The Hanbalis disagreed concerning that and were of the view that the disbeliever cannot be a mahram for a Muslim woman. See: al-Mughni (5/34).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) favoured the view of the majority of scholars and said:
If he is of a different religion, as in the case of a disbelieving man and a Muslim woman, then he may be a mahram to a Muslim woman, on condition that he can be trusted with her. If he cannot be trusted with her, then he cannot be a mahram and she cannot be allowed to travel with him. End quote.
Ash-Sharh al-Mumti‘ (7/41)