Nephew and niece

A nephew is the son of someone's brother or sister. A niece is the daughter of the person's brother or sister. To the nephew or niece, the person is their uncle or aunt. The relationship of aunt/uncle to niece/nephew is an example of second-degree relatives, meaning that their coefficient of relationship is 25%. In some cultures and families, it is common to call the child of one's cousin a "niece" or "nephew". In old English, the son of one's sister was called a "sister-son".
Detail.
A sororal or fraternal relative is one who is related through their sister or brother. For example a sororal niece is the subject's sister's child. An in-law is a relationship that is not by blood, but instead by marriage. The in-law shares the in-law relationships of the spouses relatives. For niece or nephew in-law could be the child of a spouse.