Is a Maternal Uncle a Mahram?
Question

What is the ruling on a wife’s maternal uncle visiting her when she is alone during the time her husband is working? This situation has come up more than once.
Praise be to Allah.Is My Maternal Uncle a Mahram?
The maternal uncle is a Mahram for all the daughters and granddaughters of his sisters, because Allah says in Surat An-Nisa’, defining women who are forbidden in marriage :
{Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters.} [An-Nisa’ 4:23]
Permissible Interactions with a Maternal Uncle
So there is nothing wrong with a maternal uncle visiting his sister’s daughter and being alone with her or travelling with her, so long as there are no reasons for suspicion, such as if he is an evildoer and cannot be trusted with his sister’s daughter. If there are any grounds for suspicion, then he is not allowed to be alone with or visit her when her husband is not present.
Different Scholarly Opinions
Some of the Salaf – such as `Ikrimah and Ash-Sha`bi – were of the view that even though it is not permissible for a paternal or a maternal uncle to marry their sister’s or brother’s daughter, it is nevertheless not permissible for her to show her adornment in front of them, and she must observe Hijab with them. They quoted two things as evidence for that:

Maternal and paternal uncles are not mentioned in the verse of Surat Al-Ahzab which says that it is permissible for a woman to show her adornment in front of her Mahrams. Allah Says (interpretation of the meaning):

{It is no sin on them (the Prophet’s wives, if they appear unveiled) before their fathers, or their sons, or their brothers, or their brother’s sons, or the sons of their sisters, or their own (believing) women, or their (female) slaves. And (O ladies), fear (keep your duty to) Allah. Verily, Allah is Ever All-Witness over everything.} [Al-Ahzab 33:55]
Here Allah does not mention the paternal or maternal uncles.

They said: and because maternal or paternal uncles may describe the woman to their sons.

However, the majority of scholars are of the view that maternal and paternal uncles are among the Mahrams, in front of whom it is permissible for a woman to show her adornment.
In response to the comment that the maternal uncle and paternal uncle are not mentioned in the verse, they said:
They are not mentioned because their status is like that of the parents, hence Allah calls the paternal uncle a father, in the verse in which He says (interpretation of the meaning):
{Or were you witnesses when death approached Ya`qub (Jacob)? When he said unto his sons, ‘What will you worship after me?’ They said, ‘We shall worship your Ilah (God — Allah) the Ilah (God) of your fathers, Ibrahim (Abraham), Isma`il (Ishmael), Is-haq (Isac), One Ilah (God), and to Him we submit (in Islam.)’} [Al-Baqarah 2:133]
Isma`il was the paternal uncle of Ya`qub (peace be upon them both)

They are not mentioned because it is sufficient to mention the brother’s son and sister’s son, so it is more appropriate that this ruling should apply to the paternal uncle and maternal uncle.

As-Sa`di (may Allah have mercy on him) said (p. 788):
“It is no sin on them” means if they do not observe hijab in front of them. No mention is made here of paternal uncles and maternal uncles because if women do not have to observe hijab in front of those whose paternal and maternal aunts they are, the sons of their brothers and sisters, even though they (these women) are of a higher status than them, then it is more likely that they do not have to observe hijab in front of their paternal and maternal uncles. (End quote)
With regard to the reason given, that the maternal uncle or paternal uncle may describe the woman to their sons, the majority of scholars responded to this by noting that this is a weak argument, because if this is accepted, it would imply that it is not permissible for a woman to uncover her adornment in front of any woman, because she may describe her to her sons!
One thing which indicates that the view of the majority – that it is permissible for a woman to show her adornment to her paternal and maternal uncles, and that it is permissible for them to enter upon her and be alone with her – is correct is the report narrated by Al-Bukhari (4796) and Muslim (1445) from `Aishah (may Allah be pleased with her), who said: Aflah, the brother of Abu Al-Qu`ays, asked for permission to enter upon me after the (verse of) Hijab had been revealed, and I said, I will not let him in until I ask the Prophet (peace and blessings of Allah be upon him) for permission, because his brother Abu Al-Qu`ays is not the one who breastfed me, rather the wife of Abu Al-Qu`ays breastfed me.
The Prophet (peace and blessings of Allah be upon him) entered upon me and I said to him: “O Messenger of Allah, Aflah, the brother of Abu Al-Qu`ays, asked for permission to enter upon me, but I refused to let him in until I asked your permission. The Prophet (peace and blessings of Allah be upon him) said: “What kept you from letting him in? He is your paternal uncle!” I said, “O Messenger of Allah, the man is not the one who breastfed me, rather the wife of Abu Al-Qu`ays breastfed me.” He said: “Let him in, for he is your paternal uncle, may your right hand be rubbed with dust.”
So if a paternal uncle through breastfeeding is allowed to enter upon a woman and be alone with her, then it is more appropriate that a paternal uncle through blood ties be allowed to do so, and the same applies to the maternal uncle. (End quote from Tafsir Al-Qasimi, 13/298)
For a deeper understanding, please review the following answers:
Who Is a Woman’s Mahram
A Woman’s Maternal Uncle and Paternal Uncle Are Mahram to Her and Her Daughters
The Maternal and Paternal Uncles of the Husband Are Not Mahram to the Wife
Are the Wife’s Maternal Aunt and Paternal Aunt Regarded as Mahram
And Allah knows best.