Nick and Kelly
But apparently this isn't just genetics. Here are some other possible sibs who might have been separated at birth:

Bruce Campbell as Autolycus, King of Thieves
Nick, during the off-season
(and my casting choice for Uncle Rory)

Matt Dillon
Nick doing his impression of a male model

That icky Parker Abrams
That nice Xander Harris
(Adam Kaufman)

Truman Burbank (aka Jim Carrey)
Xander, thinking this one's kinda iffy
Anyone else?
suggestions, jibes and giggles to Spike