All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not permissible to marry at the same time a woman and the daughter of her niece because the daughter of the daughter of the brother/sister is like the daughter of the brother/sister. The scholars stated that it is forbidden to marry at the same time a woman and her aunt. Ibn Qudaamah said: “…Moreover, there is no difference between a paternal aunt or a maternal aunt in reality or in a figurative sense, like the father’s paternal aunts or maternal aunts, the mother’s paternal aunts or maternal aunts and all preceding aunts (e.g. aunts of the grandparents), be it from family lineage or from suckling relationship.”
Besides, Ibn Taymiyyah said: “Combining in marriage at the same time a woman with the maternal aunt of her father or the maternal aunt of her mother, or the paternal aunt of her father, or the paternal aunt of her mother, is like combining a woman with her paternal aunt or her maternal aunt according to the view of the Muslim scholars who all agreed that this is forbidden.”
Allaah Knows best.