Payback's a B tch

AU. Dean Winchester and Castiel Novak are best friends, have been since they were kids – going all the way through college. Dean knew Cas had a sister 2 years younger than him, but as far as he remembered she was just a little squirt who was shipped off to a posh-as-hell boarding school when she turned 13. But now she's back, and she's set on causing absolute chaos in Dean's life.