Marvin and Darvin is almost cruel IMO.
Briana and
Brittany would get confusing, too. The other twin sets are nice, though. Do you remember if
Ashley and
Whitney were boys or girls?
The last sibset is pretty odd, IMO.
China and Japan? It's also confusing because there seems to be no consistency -- I don't mean themey, alliterative names, but at least in the first sibset they all are fairly traditional, English names. The last sibset is all over the board!