With the exception of identical (monozygotic) twins, triplets, etc, each child receives a different mixture of genes from each parent,
(if this were not the case all siblings would be identical).
So the children of one couple would not be identical to the children of the other couple.
Genetically these cousins would be like siblings, so they would be unusually similar cousins, but not identical.
I think Russell was referring indeed to identical twins... [;)]
http://images.google.it/imgres?imgurl=http://www.sciencemuseum.org.uk/on-line/genes/images/1-3-4-2-2-2-0-0-0-0-0.jpg&imgrefurl=http://www.sciencemuseum.org.uk/on-line/genes/118.asp&usg=__weLNVfbvEEq_xRKZdeTCRZFp7f8=&h=360&w=360&sz=23&hl=it&start=16&um=1&tbnid=cdalB4o4xOubAM:&tbnh=121&tbnw=121&prev=/images%3Fq%3D%2522identical%2Btwins%2522%26um%3D1%26hl%3Dit%26sa%3DG
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.sciencemuseum.org.uk%2Fon-line%2Fgenes%2Fimages%2F1-3-4-2-2-2-0-0-0-0-0.jpg&hash=c4b898264eb56df707a8019a6c6ccaec)
http://www.sciencemuseum.org.uk/on-line/genes/images/1-3-4-2-2-2-0-0-0-0-0.jpg