Shaykh al-Islām—may Allah have mercy on him—was asked:

A man had a concubine under a written deed, then passed away into Allah’s mercy. His son’s son subsequently married this concubine of his grandfather. Is that lawful?

He replied:

It is not permissible for him to marry his grandfather’s concubine with whom his grandfather had intercourse, by the consensus of the Muslims. If he marries her, they must be separated; he may not remain with her. If he considers this lawful, he must be called upon to repent three times. If he repents, well and good; otherwise, he is to be put to death.

The Shaykh—may Allah Most High have mercy on him—also said:

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As for the prohibition of “combining women in marriage,” a man may not be married simultaneously to two sisters, as expressly stated in the Quran, nor to a woman and her paternal aunt, nor to a woman and her maternal aunt. Neither may the older woman be married while the younger is already his wife, nor the younger while the older is already his wife. It is established in an authentic hadith that the Prophet, peace and blessings be upon him, forbade this. It is reported that he said:

“If you do that, you will sever your ties of kinship.”

Even if one of the two agrees to his marrying the other alongside her, it remains impermissible, for feelings change. This is illustrated by the report: “When Umm Habība proposed to the Prophet, peace and blessings be upon him, that he marry her sister, the Prophet, peace and blessings be upon him, asked her, ‘Would you like that?’ She replied: