Use the adjective consanguine to describe two people who are biologically related to each other. For example, a grandmother and her grandson are consanguine.
Consanguine is a fancy way to say "related." People who are connected through marriage or adoption are not consanguine, because they aren't genetically related to each other, but mothers and children, uncles and nephews, and brothers and sisters are all consanguine. A more common word with the same meaning is consanguineous, and both words come from the Latin consanguineus, which means "of the same blood."