"Double cousins", as siblings share all four grandparents. This should make them equally genetically similar.

Example: Alice and Bob are the parents of Charlotte and Dave. Eve and Frank are the parents of greg and Hannah. Now Charlotte and Grag are the parents of Ike and Juliet while Hannah and Dave are the parents of Kevin.

Ike, Juliet and Kevin share all four grandparents. Why would Ike and Juliet be more geneticaly similar than either of them and Kevin?

Double cousins share all four grand parents, as do siblings. By this simple argument double cousins should be equally genetically similar to siblings. But are they? And if they are not what gives rise to the difference?

Example: Alice and Bob are the parents of Charlotte and Dave. Eve and Frank are the parents of Greg and Hannah. Now Charlotte and Greg are the parents of Ike and Juliet, while Dave and Hannah are the parents of Kevin. Thus, Ike, Juliet and Kevin all share all four grandparents. Why would Ike and Juliet be more genetically similar than either of them and Kevin?

Does the Neanderthal has a different ancestor? And where they came from?