A son of one's cousin or cousin-in-law
A son of one's sibling, brother-in-law, or sister-in-law; either a son of one's brother (fraternal nephew) or a son of one's sister (sororal nephew).
A daughter of one’s cousin or cousin-in-law
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").