third-degree relatives

Definition of third-degree relative
National Cancer Institute

third-degree relative

(... deh-GREE REH-luh-tiv)
A first cousin, great-grandparent, great-aunt, great-uncle, great-niece, great-nephew, great-grandchild, half-aunt, or half-uncle of an individual. Also called TDR.

Source: NCI Dictionary of Cancer Terms