A man may not marry...

A man may not marry: (a) The mother or grandmothers of his wife; (b) The daughter, granddaughter, etc., of a wife with whom his marriage has been consummated; (c) The ex-wife of his son, grandson, etc.; (d) The ex-wife of his father, grandfather, etc. There are certain women whom a man may marry singly, but not at the same time. These are (a) two sisters, and (b) a woman and the sister of her mother or father.

In the second case, the Shi’a take exception to the four Sunni schools by saying that if the aunt agrees to share her husband with her niece, the contract is valid.[^8] Except for the daughter of his wife, in the three other instances of relationship by marriage, the women become forbidden to the man as soon as the marriage contract is concluded; consummation of the marriage is not necessary.

But if a man wants to marry the daughter of a wife with whom he has not consummated his marriage, he can do so if he first divorces the wife. Once the marriage has been consummated, the wife's daughter is forbidden to him forever, whether or not the marriage contract is valid. If a man should marry both a woman and her daughter or two sisters in a single contract, both marriages are invalid.

In both cases, should he first marry one and then the other, the first contract is valid and the second void. The schools of law differ as to what exactly establishes the unmarriageability of a woman as the result of a relationship by marriage. For the Hanafis, unmarriageability is established by a valid marriage contract, sexual intercourse in whatever context (i.e.

whether as the result of a valid contract, an invalid one, or fornication), love play, or looking at the private parts of a person of the opposite sex.[^9] The Shafi'is hold that unmarriageability is established only by a valid marriage contract or by the consummation of an invalid marriage contract. They do not consider any other factors, such as fornication or love play, as sufficient to establish unmarriageability.

^10 The Maliki position is the same as the Hanafi, except in the case of fornication; like the Shafi'is, the Malikis hold that no honor or respect can be paid to fornication. ^11 In the Hanbali view an invalid contract, like a valid one, results in unmarriageability, as does sexual intercourse.