A daughter of one’s sibling, brother-in-law, or sister-in-law; either the daughter of one's brother ("fraternal niece"), or of one's sister ("sororal niece").
A daughter of one’s cousin or cousin-in-law
sororal nephew
noun
The son of one's sister.
How often have the words niece and sororal nephew occurred in a corpus of books? (source: Google Ngram Viewer )