Wa alaykum salam wa rahmatuLlahi wa barakatuHu,
The Prophet (upon him be peace) said:
لا تنكح المرأة على عمتها ولا على خالتها، لا الصغرى على الكبرى، ولا الكبرى على الصغرى
“One may not marry a woman while married to her paternal or maternal aunt; and one may not marry the niece while married to the aunt, nor the aunt while married to the niece.”
In his commentary on Mukhtasar al-Muzani, Imam al-Haramayn explained the meanings of the words “kubra” and “sughra” and the translation has been rendered based on his explanation. It is unlawful for one to be married to a woman and her aunt at the same time. In the instance that the marriage to one’s wife would end, like by divorce or death, then one would be allowed to marry her niece. (Mughni al-Muhtaj 4:295; Nihayat al-Matlab 12:225; Tuhfat al-Habib 3:425)
And Allah knows best.
Answered by Shaykh Yaqub Abdurrahman