I am married to a woman and I wish to marry her niece (M.M.Q); is she lawful for me to marry?
(Part No. 18; Page No. 241)
In case you wish to marry your wife's niece, there is no harm in that after you divorce your wife and after the end of her `Iddah (woman's prescribed waiting period after divorce or widowhood). However, if you want to marry the girl while you are still married to her aunt, this is not permissible, for the Prophet (peace be upon him) stated,
“A woman should not be married together with her paternal aunt or a woman together with her maternal aunt.”
(Agreed upon by Al-Bukhari and Muslim).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.