Amazon.com can be a good place to get items, however I doubt you'd get them before Christmas ordering now unless you paid a stupid amount in shipping.

The US site sells certain food items and you could alway see if she has an Amazon wish list too.

Otherwise, food items wise there's Skyco, Cybercandy and various other American food import shops too.
Where in the states is your SIL from? Maybe try and find something regional for her? Like I'm from Cleveland, OH - you could get me the movie A Christmas Story (as no one over here would really know about it and it was written & filmed in Cleveland). Or the lamp from said movie.

Good luck!