Is the Mother's Uncle a Mahram?
Question

Is it permissible to wear Hijab in front of one’s mother’s paternal uncle and maternal uncle?
Praise be to Allah.Is a Mother’s Uncle Considered a Mahram in Islam?
A person’s maternal uncle is regarded as a maternal uncle to him and all his offspring. Similarly a person’s paternal uncle is regarded as a paternal uncle to all his offspring.
Islamic Ruling on Hijab and Interaction with Maternal Uncles
Based on that, there is nothing wrong with a woman taking off her Hijab in front of her mother’s maternal or paternal uncle ; she may shake hands with him and be alone with him, because he is a Mahram to her. Allah, may He be Exalted, says, when listing women who are Mahrams (interpretation of the meaning): {your brother’s daughters, your sisters daughter’s.} [An-Nisa’ 4:23]
Opinions from Islamic Scholars
Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni (7/90): “Sisters’ daughters and their daughters are Mahrams, because they are the daughters of the sister. The same applies to the daughters of brothers’ daughters. (End quote)
The Standing Committee for Issuing Fatwas was asked: I have a sister, and she has sons and daughters. The sons have got married and produced sons and daughters. Is it proper for me to kiss these girls, as I am the maternal uncle of their father? Similarly, the daughters of my sister have got married and produced sons and daughters. Is it proper for me to kiss these girls, as I am the maternal uncle of their mother? And is it the case that they do not have to cover in front of me?
They replied:
A man is a Mahram to the daughters of his sister’s daughters, and to the daughters of his sister’s sons, no matter how far the line of descent reaches, because he is a maternal uncle to them, and they do not have to observe Hijab in front of him, because Allah, may He be Exalted, says (interpretation of the meaning): {Forbidden to you (for marriage) are: your mothers… your brother’s daughters, your sister’s daughters} [an-Nisa’ 4:23]. That includes the daughters (of one’s brother and sister) and their offspring. (End quote from Fatawa Al-Lajnah Ad-Da’imah, 17/384)
They were also asked: Is it permissible for a woman to uncover and to greet with Salam her mother’s maternal and paternal uncles, and her father’s maternal and paternal uncles? Please also give the Fiqhi evidence proving that that is allowed or prohibited.
They replied:
It is permissible for a woman to show of her adornments to her mother’s maternal uncle and paternal uncle, and to her father’s maternal and paternal uncles, that which she may uncover in front of her Mahrams, for two reasons:

They are Mahrams to her:

It is prohibited for any of them to marry her, because of the general meaning of the verse (interpretation of the meaning): {Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your fathers’ sisters, your mothers’ sisters, your brothers’ daughters, your sisters’ daughters.} [An-Nisa’ 4:23]
What is meant by the brother’s daughters in this verse is daughters (and granddaughters) of the brother, no matter how far the line of descent reaches, not only the brother’s own daughter. The paternal uncle of a woman’s father is the brother of her grandfather, and grandfathers, no matter how far the line of ascent reaches, are (regarded as being the same as) fathers.
So this woman is included in the general meaning of the prohibition of marriage to the daughters of one’s brother. The paternal uncle of a woman’s mother is a brother to the father of her mother, so she is included in the general meaning of the prohibition of marriage to the daughters of one’s sister.
What is meant by the sister’s daughters in the verse is the daughters (and granddaughters) of the sister, no matter how far the line of descent reaches, not only the sister’s own daughter. The maternal uncle of the woman’s mother is a brother to the mother of the mother.
The maternal uncle of her father is a brother to the mother of her father. So this woman is included in the general meaning on the prohibition of marriage to the daughters of one’s sister. Therefore, once it is proven that she is one of the Mahrams of those who are mentioned in the question, it is permissible for her to uncover in front of them that of her adornments that she may uncover in front of those whom Allah mentions in the verse (interpretation of the meaning): {and not to reveal their adornment except to their husbands, their fathers, their husbands fathers…} [An-Noor 24:31]

Allah has permitted the woman to uncover in front of her brother’s sons and her sister’s sons, and their grandsons, no matter how far the line of descent reaches, that which she may uncover of her adornments in front of all her Mahrams, fathers, sons and brothers.

Allah, may He be Exalted, says (interpretation of the meaning):
{and not to reveal their adornment except to their husbands, their fathers, their husbands fathers, their sons, their husbands sons, their brothers or their brothers sons, or their sisters sons…} [An-Nur 24:31]
The maternal uncle of a woman’s mother and the maternal uncle of her father, and the paternal uncles of her mother and the paternal uncle of her father, no matter how far the line of ascent reaches, come under the heading of sons of brothers and sisters, and their grandsons, no matter how far the line of descent reaches. The ruling about showing adornment is the same with regard to all of them. As for greeting them, that is by shaking hands only.
Who Is a Woman’s Mahram
Is Brother-in-Law a Mahram
The Maternal and Paternal Uncles of the Husband Are Not Mahram to the Wife
Are the Wife’s Maternal and Paternal Aunts Regarded as Mahram
And Allah knows best.