Re:
I don't think it would harm your snake in the least to wait that long to be fed.
However, there are those among us, myself included, who always attempt to fed upon arrival. I've done this for 10 snakes, and so far no harm has been done. If you have everything in the vivarium set up beforehand, and know the temperatures are in the ok range, I don't see any problem with you attempting to feed it Wednesday night.
That way you'll know it ate, and your sister won't have to worry about feeding it. But if it doesn't eat, I don't think it will be harmed by not being fed until you return. Snakes are pretty hardy creatures, and can go quite a while without food.
So just take a deep breath, relax and when your snake arrives on Wednesday...attempt to feed it in the deli-cup before you even put it in it's viv and once it eats, put it in it's new home and it's ready to go. If it doesn't eat right away, don't worry. There are a few of us struggling with our own hatchlings that are going on a month or more of not eating.
Hopefully things go alright for you. =)