A sibling-in-law is the spouse of one's sibling, the sibling of one’s spouse, or the spouse of one's spouse's sibling. In Indian English the latter can be referred to as a co-sibling (specifically a co-sister, for the wife of one's sibling-in-law, or co-brother, for the husband of one's sibling-in-law).
More commonly, a sibling-in-law is referred to as a brother-in-law for a male sibling-in-law and a sister-in-law for a female sibling-in-law.
In the lineal kinship system used in the English-speaking world, a niece or nephew is a child of an individual's sibling or sibling-in-law. A niece is female and a nephew is male, and they would call their parents' siblings aunt or uncle. The gender-neutral term nibling has been used in place of the common terms, especially in specialist literature.
As aunt/uncle and niece/nephew are separated by one generation, they are an example of a second-degree relationship. Unless related by marriage, they are 25% or more related by blood if the aunt/uncle is a full sibling of one of the parents, or 12.5% if they are a half-sibling.