How old is your sister, that will perhaps have some bearing on the outcome of new vs used.
I think this is perhaps a generation thing, older generations felt lucky to get a couple of things new or second hand for birthdays and christmases, the current generation is truly distraught if they don't get the latest console with a stack of the latest games and a new bike and a...
.. You get my drift?
If your sister is of the correct mentality, I would imagine if you are able to find a 5D kit in its original box your sister would be made up especially if during the run up to Christmas you subtly ask her thoughts on full frame and crop and then explain, again with a degree of subtlety, what advantages full frame full frame has over crop.
I do feel it is important that the gift should look like new so the kit in original box manual leads perhaps a new battery etc.
I think you will be doing her the favour of removing one step of upgrade.
Of course the new camera even a xxxD is guaranteed to be a success.
The thing is you should know the answer to this before you buy a camera as disappointment is a double edged sword and it will spoil your day too if your sister is disappointed.