First-degree relative
A first-degree relative is a person's parent, full sibling or child. It constitutes a category of family members that largely overlaps with the term nuclear family, but without spouses.
If the persons are related by blood, the first degree relatives share approximately 50% of their genes. First-degree relatives are a common measure used to diagnose risks for common diseases by analyzing family history.
Marriage or sexual relations between first-degree relatives falls within the definition of incest.
If a parent is an identical twin, then the aunt or uncle who is the other twin, is also considered a first-degree relative because being genetically identical to the parent ultimately makes them the children's parent as well, genetically speaking.
