Marriage is a Sunnah that is strongly recommended. Muslims are encouraged to marry as it is regarded as half of one’s deen. As with everything in life, Islam has given us guidance on how to marry, what to look out for in a potential spouse and who to marry. This article will be focusing on who you cannot marry.
True, there are a lot of fish in the sea, but there are some fish you must not catch! Not to worry, these fish are not many. Knowing who you cannot marry also helps you know your mahram. Mahram are the people you are permanently forbidden from marrying due to blood or marriage ties.
The people Muslims are not allowed to marry are divided into 2 broad groups.
1. Permanent prohibition
2. Temporary prohibition
These are the people a Muslim is permanently forbidden from marrying. These are also called his/her mahram. A Muslim woman is allowed to uncover in front of these people and does not have to wear hijab. This category is further sub-divided into 3;
a. Blood relatives
b. In-law based relationships
c. Foster relationships
These are people prohibited to marry because of the blood relationship with them. Allah prohibited marriage to the following women ;
· Mother – this also includes every woman that was a direct cause of your birth like your paternal and maternal grandmothers, great-grandmothers, great great grandmothers and upwards.
· Sister – this includes your half-sisters and step sisters. Half-sisters are the ones you share a parent with, i.e. you have the same mother or father. While step sisters are the ones you don’t share any parent with e.g. your father married a widow who already had a daughter, you don’t share the same father or mother with that girl so she’s your step sister.
· Daughter – this includes every woman whose birth was a directly caused by you like your granddaughter, great granddaughter and downwards.
· Mother’s Sisters (maternal aunts) – this also includes your grandmother’s sisters from both mother and father’s sides (i.e. grandaunts) and upwards.
· Father’s Sisters (paternal aunts) – this also includes your grandfathers’ sisters from both mother and father’s sides and upwards.
· Brothers’ daughters (nieces) – this also includes your nieces daughters and downwards.
· Sisters’ daughters (nieces) – this also includes your nieces daughters and downwards.
These are people prohibited to marry because they have marriage ties with some of your family members.
· Father’s other wives i.e. your step mothers
· Sons’ wives (daughters-in-law)
· Wives’ mothers (mothers-in-law)
· Wives’ daughters from other men (i.e. your step daughters) – this prohibition holds only after consummation with their mothers. If the mother is divorced or dies before the marriage was consummated, her daughter (from another man) is lawful for you to marry.
Foster mothers and foster fathers are taken as real mothers and fathers when it comes to marriage laws; hence, all the people listed under the Blood Relatives category are also listed here. A foster mother is the woman (human being) who breastfed you 5 times while you were below 2 years. While a foster father is the one who caused the milk to flow from that foster mother. The present husband of the woman isn’t always the foster father as she might have re-married after the one who impregnated her (caused the milk to flow) divorced her or died.
‘Aa’ishah narrated: “When the Qur’an was first revealed, the number of breast-feedings that would make a child a relative (mahram) was ten, then this was abrogated and replaced with the number of five which is well-known.”
Umm Salamah said: The Messenger of Allah (ﷺ) said: “The only breastfeeding that creates the relationship of mahram is that which fills the stomach from the breast, before weaning.”
During the time of the Prophet (ﷺ) 2 people were married and a slave woman came to say she breastfed both of them. The Prophet (ﷺ) nullified the marriage even though they already had kids. 
The daughter of Hamzah (the Prophet’s uncle) became his niece (instead of his cousin) because the same woman breastfed the Prophet and Hamzah. Hence they were like brothers. 
In summary, the conditions for foster mother are;
· She must be a human being, not an animal. If a baby drinks an animal’s milk, the animal does not become its foster mother.
· The child must be below 2 years. If a husband drinks his wife’s milk, she doesn’t become his foster mother.
· The suckling must be done a minimum of five known times to the child’s fill. If the child was not satisfied in any of the suckling or was fed less than 5 times, then the woman does not qualify as a foster mother.
These are people prohibited to marry temporarily because of some conditions. Once those conditions are removed, marriage to the person becomes lawful.
a. Wives’ Sisters – Allah makes it forbidden to marry a woman and her sister at the same time  i.e. you cannot marry your sister-in-law. But if the woman is divorced or dies, then her sister becomes lawful to marry. This prohibition also extends to her aunts and nieces.
b. Non-Muslims – Allah has prohibited Muslims from marrying non-Muslims.  Once any reverts to Islam, they become lawful to marry. The exception to this are the Jewish and Christian women lawful for Muslim men to marry on the condition that they are virgins. 
c. One who commits zina – Allah has prohibited marrying a Muslim who commits zina.  This is temporary until the person repents.
d. A Muslim man who has 4 wives is temporarily not allowed to marry. Allah has placed the maximum number lawful for a man as 4. If one of his wives dies or is divorced, then he may marry another.
e. A Muslim in the state of Ihram during Hajj or Umrah is temporarily not allowed to marry till he/she leaves the state of Ihram. This is because the Messenger of Allah (ﷺ) said “A muhrim must neither marry himself, arrange the marriage of another one, nor should he make the proposal of marriage.” 
f. Every married woman is temporarily forbidden to marry and be married till she is widowed or divorced. 
g. Women in their iddah – Allah has also prohibited women temporarily from marriage till they complete their iddah period. 
Now that we know the fish that are not permissible in the sea, every other person apart from all these are permissible for marriage. May Allah grant us spouses that will be the coolness of our eyes. Aammeen.
1. An-Nisa’a 4:23
2. An-Nisa’a 4:22
3. Sahih Bukhari: Book 7, Volume 62, Hadith 166
4. Muslim: Book 8, Hadith 3421
5. Al-Tirmidhi: Book 12, Hadith 7. Classed as saheeh by al-Albaani in al-Irwa’, 2150
6. Sahih Bukhari: Book 3, Volume 48, Hadith 828
7. Sahih Bukhari: Book 3, Volume 48, Hadith 813
8. Al-Baqarah 2:221
9. Al-Maida 5:4
10. An-Nur 24:3
11. Sahih Muslim, Book 8, Hadith 3278
12. An-Nisa’a 4:24