second-degree relative
someone who shares 25% of a person's genes: uncles, aunts, nephews, nieces, grandparents, grandchildren, half-siblings, and double cousins
half-sibling
one of two or more individuals having one parent in common
grandparent
parent's parent
niece or nephew
child of a sibling or half-sibling
grandchild
child's child. Avoid using with "relative" (P1038): add item for child instead and list there (both with "child" (P40))
uncle or aunt
parent's sibling or parent's sibling-in-law
double cousin
child of one's paternal aunt or uncle with one's maternal uncle or aunt