Wiktionary
second-degree relative
n. One who shares 25% of your DNA such as a grand-parent, grand-child, aunt, uncle, nephew, niece, or half-sibling.
Wikipedia
Second-degree relative
A second-degree relative (SDR) is someone who shares 25% of a person's genes. It includes uncles, aunts, nephews, nieces, grandparents, grandchildren, and half-siblings.