Who is a Mahram?
Answer : As a general principle, it is worth remembering that a Mahram is one with whom marriage is permanently unlawful. This is the reason why “Mahram” is translated in English as unmarriageable kin.
This (permanent prohibition of marriage) is established in three ways: By kinship (qarabah), foster relationship (radha’a) and relationship through marriage (sihriyya).
It is stated in the famous Hanafi Fiqh treatise, al-Hidaya:
“A Mahram (for a woman) is he, between whom and her marriage is permanently unlawful, whether this is due to the relationship of lineage/kin (nasab) or because of some other reason, such as foster relationship (radha’a) or relationship by marriage (musaharah).” (al-Hidaya, Kitab al-Karahiyya, 4/461-462)
Imam al-Kasani (Allah have mercy on him) states:
“A Mahram is he, with whom marriage is permanently unlawful, either by kinship, foster relationship or relationship by marriage.” (Bada’i al-Sana’i, 2/124)
Thus, permanent unlawfulness of marriage is established with the above-mentioned three types of relationships, and a Mahram is he with whom marriage is unlawful permanently. In other words, one becomes a Mahram due to these three types of relationships.
Let us now look at these relationships in detail
Relationship of family/lineage (qarabah)
It is permanently unlawful for a man to marry the following (hence he will be considered a Mahram for them):
a) Mother, grandmother, and on up;
b) Paternal grandmother, and on up;
c) Daughters, grand daughters, and on down;
d) All type of sisters (whether full or half),
e) Maternal and paternal aunts,
f) Nieces (brother’s or sister’s daughters),
Allah Most High says:
“Prohibited to you (for marriage) are: Your mothers, daughters, sisters, father’s sisters, mother’s sisters; brother’s daughters, sister’s daughters….” (Surah al-Nisa, 22)
Thus, besides the abovementioned relatives, marriage with others relative will be lawful, thus they will not be considered to be mahrams, such as cousin brothers, cousin sisters, mother’s sister’s husband, etc.
Relationship of fosterage (radha’a)
Whosoever is a Mahram through the relationship of lineage, will also be considered a Mahram by fosterage.
Allah Most High states further along in the same verse mentioned above:
“And (prohibited to you in marriage are) your foster-mothers and foster-sisters.” (Surah al-Nisa, 23)
Sayyiduna Abd Allah ibn Abbas (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace) said about Hamza’s daughter: “I am not legally permitted to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster-brother.” (Sahih al-Bukhari, no. 2502)
Therefore, the relationships that are unlawful through blood and lineage will also be unlawful through fosterage. As such, a foster-father (foster mother’s husband), foster-brother, foster-uncle, foster-nephew, etc will all be considered to be a woman’s Mahram, and one will be a Mahram to a foster-mother, foster sister, foster niece, etc.
However, one should remember that this is only when breastfeeding takes place in the period designated for it, which is two and a half years (according to Imam Abu Hanifa) and two years (according to Abu Yusuf and Imam Muhammad).
Sayyida A’isha (Allah be pleased with her) reports: “Once the Messenger of Allah (Allah bless him & give him peace) entered my house while a man was with me. He said: “O A’isha! Who is this?” I replied: “My foster-brother” He said: “O A’isha! Be careful in determining who your foster-brother is, for suckling is only valid if it takes place in the suckling period”. (Sahih al-Bukhari, no. 2504 & Sahih Muslim, no. 1455)
One should be careful in determining who is a Mahram through foster relations, for determining this, at times, can be complex and complicated. One must refer to a scholar before coming to a judgment.
3) Relationship of marriage (sihriyya or musahara)
The third relationship with which marriage becomes permanently unlawful and consequently the relationship of being a Mahram (mahramiyya) is established is that of marriage.
There are four types of people with whom marriage becomes unlawful permanently due to the relationship of marriage:
a) One’s wife’s mother (mother in-law), grandmother and on up: Marriage with her becomes unlawful by merely contracting marriage with the daughter, regardless of whether the marriage was consummated or otherwise.
b) One’s wife’s daughter (from a previous marriage), grand-daughter and on down: Marriage with her becomes unlawful (permanently) if the marriage with her mother was consummated.
Allah Most High mentions both these situations in the same verse quoted earlier:
“And (prohibited to you in marriage) are your wives’ mothers; your step-daughters under your guardianship, born of your wives with whom you have had sexual intercourse. There is no prohibition if you have not cohabited.” (Surah al-Nisa, 22)
Also included in the above will be one’s wife’s son’s (stepson’s) daughter, for she is also considered to be a stepdaughter (rabiba).
c) The wife of one’s son, grandson, and on down: This is regardless whether the son consummated the marriage or otherwise. Allah Most High says in the same verse:
“And (prohibited to you in marriage) are (those who have been) wives of your sons proceeding from your loins.” (ibid)
The verse specifically refers to one’s real sons, thus marriage with one’s foster son’s wife will be permissible, if he was to divorce her.
d) One’s stepmother, step grandmother and on up: Meaning those women who have been in the marriage of one’s father or paternal or maternal grandfather.
Allah Most High says:
“And marry not women whom your fathers married, – except what is past: It was shameful and odious.” (Surah al-Nisa, 21)
To sum up, a Mahram is he with whom marriage is permanently unlawful, and this permanent unlawfulness/prohibition of marriage is established in three ways: The relationship of lineage, relationship through fosterage and the relationship through marriage.
In light of the above explanation, your question will have been answered. Nevertheless, If your mother breastfed this nephew of hers when he was a baby (meaning, within the first two or two and a half years of the child’s age), then the rules of fosterage (radha’a) will be applied, in that you and your other sisters will not have to observe Hijab from him when he reaches puberty, neither will marriage be permissible between him and any of your sisters. He will be considered a Mahram to your mother, yourself and all your sisters.
The ruling will be similar if you suckled him when he was young (I am not aware if you are married, thus I am only mentioning one the possibilities, given the fact you state that you looked after him since he was a baby). He will be considered your foster son, thus there will be no Hijab between him and yourself, your sisters and your mother when he reaches puberty.
However, if no breastfeeding took place (neither by your mother or your self) then merely adopting him will not remove the rules of Hijab. If the adoptive mother does not breastfeed the adopted child, then the relationship of fosterage will not be established and the child will be classed as other children with regards to Nikah and Hijab. An adopted child can marry its adoptive parents and their children. Also if a male child is adopted by a woman, she will have to observe Hijab from him after he reaches the age of puberty and visa versa. The adopted child will also (after puberty) observe Hijab with the adoptive parent’s children.