Tafsir of verse 23
Al-Mukhtasar
Allah has prohibited you to marry your mothers and grandmothers, your daughters and granddaughters, your sisters and half-sisters, your aunts and great aunts, your nieces and their children, your milk-mothers who nursed you and your sisters through nursing, the mothers of your wives (whether you have consummated the marriage or not), and the daughters of your wives by someone else (who were mostly raised in your houses, and also those who were not, if you have consummated the marriage; but if you have not, then there is nothing wrong with marrying their daughters). You are also prohibited to marry those who have been married to your own sons, even if they did not consummate the marriage, and this includes the wives of your sons through milk-kinship (nursing). It is prohibited for you to marry two sisters who are related by blood or by nursing, except if this happened in the time of ignorance before Islam, in which case Allah has forgiven it. Allah is Forgiving and Compassionate to His servants who repent to Him. In the traditions of the Prophet it is also confirmed that it is prohibited to marry a woman and her aunt at the same time.
Tafsir of the whole Surah An-Nisa
Other translations of verse 23
It is prohibited for you [to marry] your mothers, your daughters, your sisters, your paternal and maternal aunts, your brother’s daughters, your sister’s daughters, your foster mothers, your foster sisters, your mothers-in-law, your step-daughters under your guardianship if you have consummated the marriage with their mothers – however if you have not consummated the marriage, it is not a sin [to marry them] – nor wives of your own sons, nor two sisters together, except for what happened in the past, for Allah is All-Forgiving, Most Merciful.
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, your foster mother who gave you suck, your foster milk suckling sisters, your wives’ mothers, your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allâh is Oft-Forgiving, Most Merciful.
Forbidden unto you are your mothers, and your daughters, and your sisters, and your father's sisters, and your mother's sisters, and your brother's daughters and your sister's daughters, and your foster-mothers, and your foster-sisters, and your mothers-in-law, and your step-daughters who are under your protection (born) of your women unto whom ye have gone in - but if ye have not gone in unto them, then it is no sin for you (to marry their daughters) - and the wives of your sons who (spring) from your own loins. And (it is forbidden unto you) that ye should have two sisters together, except what hath already happened (of that nature) in the past. Lo! Allah is ever Forgiving, Merciful.
Prohibited to you (For marriage) are:- Your mothers, daughters, sisters; father's sisters, Mother's sisters; brother's daughters, sister's daughters; foster-mothers (Who gave you suck), foster-sisters; your wives' mothers; your step-daughters under your guardianship, born of your wives to whom ye have gone in,- no prohibition if ye have not gone in;- (Those who have been) wives of your sons proceeding from your loins; and two sisters in wedlock at one and the same time, except for what is past; for Allah is Oft-forgiving, Most Merciful;-
Forbidden to you are your mothers and your daughters and your sisters and your paternal aunts and your maternal aunts and brothers' daughters and sisters' daughters and your mothers that have suckled you and your foster-sisters and mothers of your wives and your step-daughters who are in your guardianship, (born) of your wives to whom you have gone in, but if you have not gone in to them, there is no blame on you (in marrying them), and the wives of your sons who are of your own loins and that you should have two sisters together, except what has already passed; surely Allah is Forgiving, Merciful.
Prohibited to you are your mothers and your daughters, and your sisters, and your paternal aunts, and your maternal aunts, and (your) brother's daughters, and (your) sister's daughters, and your mothers who have given suck to you, and your suckling sisters, and your women's mothers, and your step-daughters who are in your laps (i.e. under your guardianship) being born of your women whom you have been into-yet, in case you have not been into them, (Literally: entered with) it is no fault in you-and the lawful (spouses) of your sons who are of your loins, and that you should take to you two sisters together, except what has already gone by. Surely Allah has been Ever-Forgiving, Ever-Merciful.