Niece and nephew

In the lineal kinship system used in the English-speaking world, a niece or nephew is a child of the subject's sibling or sibling-in-law. The converse relationship, the relationship from the niece or nephew's perspective, is that of an aunt or uncle. A niece is female, while a nephew is male, with the term nibling used in place of the common, gender-specific terms in some specialist literature.[1]

As aunt/uncle and niece/nephew are separated by two generations they are an example of second-degree relationship and are 25% related if related by blood.