Ruling on marrying a woman and her maternal aunt’s daughter

Question:
I have three wives—praise be to Allah—and one of them has a maternal aunt’s daughter. I wish to marry her as a fourth wife. Is there any harm in that, even though both of them consent?

Answer:
There is no harm in combining your wives with their maternal aunt’s daughter, or their paternal aunt’s daughter. There is no harm in combining the daughters of two paternal uncles, or the daughters of two maternal uncles, or the daughters of two maternal aunts, or the daughters of two paternal aunts. There is no blame in this.
What is prohibited is combining two sisters. This is what is prohibited, as Allah the Exalted said: [Surat al-Nisa: 23]. It is forbidden.
Likewise, combining a woman with her paternal aunt, or a woman with her brother’s daughter, or a woman with her maternal aunt, or her sister’s daughter, is prohibited. It is not permissible to combine them, because the Messenger of Allah prohibited this. He prohibited combining a woman with her paternal aunt, and a woman with her maternal aunt. The younger sister is not to be married while the elder is still married, nor the elder while the younger is still married.
So the conclusion: You must not combine two women, one of whom is the paternal aunt of the other, or the maternal aunt of the other, even if the aunt is distant and high up in the lineage, or a maternal aunt who is not a close relative. It does not have to be a close paternal aunt; even a distant paternal aunt is included. One does not combine a woman with her father’s paternal aunt, or her mother’s paternal aunt, or her grandfather’s paternal aunt, or her grandmother’s paternal aunt. Likewise, one does not combine a woman with her maternal aunt, or her mother’s maternal aunt, or her father’s maternal aunt, or her grandmother’s maternal aunt. Her father’s maternal aunt is her maternal aunt, the woman’s maternal aunt, her father’s maternal aunt, and her mother’s maternal aunt is her maternal aunt. The Prophet (peace and blessings be upon him) prohibited combining a woman with her paternal aunt, and a woman with her maternal aunt, just as Allah prohibited in His great Book combining two sisters.
As for combining the daughters of two paternal uncles, the daughters of two maternal uncles, the daughters of two maternal aunts, and the daughters of two paternal aunts, there is no harm in that.