Can a disbeliever be regarded as a mahram for his Muslim sister when travelling?

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:

1.    

Breastfeeding

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)