2 or 3 days in transit should be no problem for large rhizomes. I don't think you have to worry too much about either rotting or dessication unless they are exposed to some fairly extreme conditions. We sell bare-root rhizomes of these types of plants in plastic bags (with the cut stem NOT enclosed-- that is where most of the rot would set in). Since it takes us about a week to process all the plants for a sale, they're in bags and packed in boxes for several days, and although the cut ends may dry out a bit, it doesn't really hurt anything. We used to dip them in a fungicide/insecticide mix, and put moist sphagnum moss in the bags, but we stopped doing both, as it didn't really seem to make much of a difference.

You don't say where your sister lives, and if there's any agricultural restrictions on shipping to her area, but if there are, that may require you to treat the plants first. You can call your local dept. of agriculture to find out. Obviously you'll need to wash them clean, and cut off all the roots too.

The main thing is to pack them tight so there's no jostling around, which can damage the growing points. An alternate method to individual bagging is to first line the box with plastic-- like maybe a large trash bag-- and then put in either crumpled newspaper or styrofoam pellets, and nest all the rhizomes inside, surrounded by the packing material, then fold the plastic over the top and seal the box. This is probably the easiest way. I wouldn't wet the newspaper as that's more likely to cause rot, although a light misting probably wouldn't hurt. If they're fresh rhizomes tightly packed, they should be just fine for a couple of weeks.