Niece
In the language of kinship, a nephew is a son of a person’s sibling, and a niece is a daughter of a person’s sibling. Conversely, that person is the aunt or uncle of their niece or nephew. The relationship of aunt/uncle to niece/nephew is an example of second-degree relatives, meaning that their coefficient of relationship is 25%.
The terms are also used colloquially for sons and daughters of siblings-in-law, even though there is no blood relationship.
Nephew
In the language of kinship, a nephew is a son of a person’s sibling, and a niece is a daughter of a person’s sibling. Conversely, that person is the aunt or uncle of their niece or nephew. The relationship of aunt/uncle to niece/nephew is an example of second-degree relatives, meaning that their coefficient of relationship is 25%.
The terms are also used colloquially for sons and daughters of siblings-in-law, even though there is no blood relationship.
Niece (noun)
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”).
“My niece just celebrated her 15th birthday.”
Nephew (noun)
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).
Nephew (noun)
A son of one’s child, grandson.