Is it permissible to marry the daughter of a paternal niece?

You are not permitted to marry the daughter of your sister's daughter, because the sister's daughter and the daughters of the children of the brother and sister—no matter how far down the lineage—are unanimously forbidden to marry. They are among the seven categories of women forbidden by lineage, whether the brother is a full brother, a paternal half-brother, or a maternal half-brother.