Agreed: in the case of siblings you share all ancestors; with cousins only some. In that way the distinction makes sense. The other way to look at it though i

Agreed: in the case of siblings you share all ancestors; with cousins only some. In that way the distinction makes sense. The other way to look at it though is that with siblings your common ancestor is one generation up. With first cousins, it's two. With second, it's three. So it's the same thing, just a matter of degree.