| Who is it? | Relationship | Common Ancestors |
|---|---|---|
| children of a husband and wife | siblings - brothers and sisters | same parents |
| children of siblings | first cousins | same grandparents without being siblings |
| children of first cousins | second cousins | same great grandparents without being siblings or first cousins |
| children of second cousins | third cousins | same great great grandparents without being siblings or first cousins or second cousins |
Note that we could speak of zeroth cousins instead of siblings. A sibling relationship is exactly the same kind of relationship as is a cousin relationship. It's just that siblings are one generation closer than are first cousins.