The above-mentioned question is ambiguous. Due to the ambiguity of the question two situations arise;
1) marrying the niece while having her aunty in Nikah
2) marrying the niece when her aunty is out of the Nikah.
With regards to the first situation it is prohibited for one to keep in Nikah aunty and niece at the same time as it is prohibited to keep sisters in Nikah at the same time, while in the second situation there are no preclusions from making Nikah to the niece.
Allah and His Rasool know best.
Islamic Lifestyle Solutions