Two people are full relatives when their link runs through a couple: two parents, or two grandparents, shared as a pair. They are half relatives when the link runs through one person only.
Half-siblings
Half-siblings share one parent. The other parent differs, often because a parent had children with more than one partner. They are one generation from the shared parent on both sides, the same counts as full siblings, but they share on average about a quarter of their DNA instead of a half.
Half-cousins and the rest
The same idea carries up the family. First cousins normally share a pair of grandparents. Half first cousins share only one grandparent, typically because two siblings of the parents' generation are themselves half-siblings. The counting of cousin degree and removal is the same, and only the shared-ancestor count changes. The theoretical DNA average halves.
Who counts?
- Step-siblings share no ancestor. They are related by a parent's marriage, so they are not half relatives and Kinage treats the link as non-blood.
- Adopted siblings are siblings in law and in family life. Kinage uses the adoptive link for naming, but marks that no biological DNA is expected across it.
- Double cousins arise when two siblings marry two siblings. Their children share both sets of grandparents through both parents, so they are cousins twice over, with the DNA of half-siblings.
How Kinage tells
When you pick two people on a tree, Kinage looks at the nearest shared ancestors. One shared person means half, a couple means full. When you describe a route yourself, use the half-sibling step to say that the link passes through one parent. See in-laws and step relatives for links that carry no blood at all.