Are the daughters of the wet nurse's husband, who breastfed the child five full, satisfying times, forbidden to the child [in marriage]?

It is forbidden for a child to marry the daughters of his wet-nurse's co-wife if these daughters are from the man whose milk the child drank. This is referred to as "milk of the stallion" (laban al-fahl). Ibn Abbas supported this prohibition with his statement, "The impregnation is one." Evidence for this is found in the hadith (definition) of Aisha (may Allah be pleased with her) regarding Abu al-Qa'is's brother, when the Prophet (peace and blessings of Allah be upon him) said to her, "Permit him to enter, for he is your uncle."