Who Is a Woman’s Mahram?
Question

Who are the mahrams in front of whom a woman can uncover?
Praise be to Allah.It is permissible for a woman to take off her hijab in front of her mahrams.
Who is a woman’s mahram?
A woman’s mahram is a person whom she is never permitted to marry because of their close blood relationship (such as her father, grandfather, great-grandfather, etc., and her son, grandson, great-grandson, etc., her paternal and maternal uncles, her brother, brother’s son and sister’s son), or because of breastfeeding (such as the brother and husband of the woman who breastfed her), or because they are related by marriage (such as the mother’s husband, the husband’s father, grandfather, etc., and the husband’s son, grandson, etc.). More details on this subject are given below:
Mahrams by ties of blood
These are the ones mentioned in Surah al-Nur, where Allah says (interpretation of the meaning):
“… and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons…” [al-Nur 24:31]
The mufassirin said: the woman’s male mahrams by ties of blood, as stated clearly in this verse or inferred by it are as follows:

the woman’s forefathers, no matter how far back the line of ascent goes through her father and her mother, such as her father’s forefathers and her mother’s forefathers. As for her husband’s forefathers, they are her mahrams by marriage, as we shall see below.
her sons, which includes her children’s children, no matter how the line of descent goes and whether they are descended from males or females, such as her sons’ sons and her daughters’ sons. As for her “husband’s sons” mentioned in the ayah, these are the husband’s sons from other wives, and these are her mahrams by marriage, not by blood, as we shall see below.
her brothers, whether they are her brothers through both the mother and father, or through the father only or the mother only.
the children of her siblings, whether they are descended through the males or females, such as the sons of her sister’s daughters.
paternal uncles and maternal uncles. They are mahrams by blood even though they are not mentioned in the ayah, because they are like parents and are regarded by people as having the same status as parents, and a paternal uncle may be called a father. Allah 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), Ishaq (Isaac)…” [al-Baqarah 2:133]. Isma’il was the paternal uncle of the sons of Ya’qub.
(Tafsir al-Razi, 23/206; Tafsir al-Qurtubi, 12/232, 233; Tafsir al-Alusi, 18/143; Fath al-Bayan fi Maqasid al-Quran by Siddiq Hasan Khan, 6/352)
Mahrams by breastfeeding
A woman may have mahrams through rada'ah. It says in Tafsir al-Alusi:
“The relationship of mahram which permits a woman to show her adornments may be through rada'ah as well as through blood ties, so it is permissible for a woman to show her adornments to those who are her fathers or sons through rada'ah.” (Tafsir al-Alusi. 18/143) 
The relationship of mahram by rada'ah is like the relationship of mahram by blood – it means that marriage is forever forbidden by virtue of that relationship of mahram. This was the view stated by Imam al-Jassas when he commented on this verse. He said (may Allah have mercy on him):
“When Allah mentioned the fathers and that their marriage to these women is forbidden forever, this indicates that the same prohibition applies in other relationships of mahram, such as the mother of the woman and those who are mahrams by rada'ah, etc.” (Ahkam al-Quran by al-Jassas, 3/317)
The same things are made haram by rada'ah as by ties of blood.
It also says in the Sunnah: “The same things are made haram by rada'ah as by ties of blood.” 
This means that the people who are mahram to a woman because of blood ties are also mahram because of rada'ah. It was reported in Sahih Muslim that ‘Aishah Umm al-Muminin (may Allah be pleased with her) said that Aflah the brother of Abu Qu’ays came and asked permission to see her. He was her uncle through rada'ah. This was after the hijaab had been revealed, so she refused to give him permission. When the Messenger of Allah (peace and blessings of Allah be upon him) came, she told him about what she had done and he told her to give him permission. (Sahih al-Bukhari bi Sharh al-'Asqalani, 9/150)
This hadith was also narrated by Imam Muslim, where the wording is: from ‘Urwah from ‘Aishah, who told him that her uncle by rada’ah, who was called Aflah, asked permission to see her and she did not let him. She told the Messenger (peace and blessings of Allah be upon him) and he said to her, “Do not observe hijab in front of him, because the same relationships of mahram are created by rada'ah as by blood ties.” (Sahih Muslim bi Sharh al-Nawawi, 10/22)
A woman’s mahrams through rada'ah are the same as her mahrams through blood ties
In accordance with the Quran and Sunnah, the fuqaha have stated that a woman’s mahrams through rada'ah are the same as her mahrams through blood ties. It is permissible for her to display her adornments before her mahrams by rada'ah just as it is permissible for her to display her adornments before her mahrams by blood ties. It is permissible for her mahrams by rada'ah to see whatever of her body it is permissible for her mahrams by blood ties to see.
Mahrams by marriage
A woman’s mahrams by marriage are those whom it is forever forbidden to marry, such as the father’s wife, the son’s wife or the wife’s mother. (Sharh al-Muntaha, 3/7)
The mahram by marriage of the father’s wife is his son from another wife, for the wife of the son it is his father, and for the mother of the wife it is the husband. Allah says in Surah al-Nur (interpretation of the meaning):
“… and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons…” [al-Noor 24:31]. 
Their husband’s fathers and their husband’s sons are mahrams of the woman by marriage. Allah mentioned them along with their (the women’s own) fathers and sons, and made them all the same in the sense that women may display their adornments in front of them. (Al-Mughni, 6/555)
For more, please check this category: Mahram relatives
And Allah knows best.