PROHIBITED DEGREES OF MARRIAGE.
According to the Qur’an they are seven: 1, mother; 2, daughter; 3, Sister; 4, paternal aunt; 5, maternal aunt; 6, sister’s daughter: 7, brother’s daughter. And the same with regard to the other sex. It is also unlawful for a Muslim to marry his wife’s sister (see Lev. xviii. 18) or his wife’s aunt during the lifetime of his wife. Foster-age in Muslim law established relationship. and therefore a foster-sister or a foster-brother is unlawful in marriage. [MARRIAGE.]
Based on Hughes, Dictionary of Islam