1. As nouns the difference between Cousin and niece is that Cousin is the son or daughter of a person’s uncle or aunt; a first Cousin while niece is a daughter of someone’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").

2. As nouns the difference between nephew and Cousin is that nephew is 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) while Cousin is the son or daughter of a person’s uncle or aunt; a first Cousin.