i know you probably mean well but most GC dont work across the border leaving you to either drive across the line to buy or just use an etransfer, which i believe you can do now to the USA, I have the same issue and honestly because of shipping ect all we ended up doing was passing GC back and forth helping out the postal service but in the end didnt end up feeling like a gift so now we only exchange gifts when we see each other and then i buy a larger gift as i dont buy often and we save our money for phone calls, maybe it wont work for you but thats what we chose to do
hope you and your sister have a Merry Christmas