Well, I tend to steer clear of anything that sounds too 'themed' if you know what I mean? Love the idea of a diner, but not if it was themed to every last details, like rollerskating waitresses with pointy boobs and coke floats on a tray.
As for the name, would your sister's first or surname work as these can sound great with the right name. Or maybe something around an American city, or an iconic landmark? Having said that, Route 66 or Liberty instantly makes me think of a 1980s nightclub, so maybe avoid them.
How about Stateside? Or take a word that makes you think of America, like Hamptons? Or if it's fusion of British and American, how about Transatlantic? Or doing something numerically like work out the shortest time difference or the mileage between UK and USA.
Ooh, or Little Apple. Or New England? Oooh, I like that one.
Hmmm, I've just spent 10 mins doing this instead of working on stuff for a client urgently. Oooops. For what it's worth I like New England (blend of the two without sounding naff) and Hamptons (has a USA feel but short and classic).
Good luck!