The wife of a person's nephew is the individual's niece-in-law. It is accepted by some that in-laws are either direct relatives of someone's spouse or the spouses of someone's direct relative, but the relationship goes no further. Others suggest that the spouse of an in-law is also considered an in-law.
The term in-law specifies that the family relationship exists through marriage rather than birth. If saying niece- or nephew-in-law feels too formal, calling any person of a younger generation a niece or nephew is acceptable as an honorary recognition. Alternately, young people may call someone older by the honorary title of aunt or uncle. This practice is commonplace in some parts of the United States and in certain cultures.