Have never heard them called that. They are not located near each other (CG at Contemp, FF at Boardwalk) and not themed similarly, food or otherwise (CG is California/American food, while FF is, obviously, more seafood oriented). They are certainly more alike than, say, Jiko and Artist Point (as an example), but not sure why they would be called "sister" restaurants.
They are both great restaurants. As for which is better, it is a matter of personal preference.