CONGRATULATIONS!!!! Everyone I have ever met who has gotten a kidney transplant feels so much better afterwards, me included, so this should be an awesome experience for you. Also congrats on have such an amazing sister. 🙂
I had my transplant at Mayo in Rochester and everything I needed while I was in the hospital was provided, even unattractive non-slip sock slippers. In my opinion you really don't need much as far as clothing while you are there. I kept all of my clothes in the small duffel bag I brought until I was ready to leave which was good because I certainly didn't want them coming home with hospital germs on them. I knew in advance that my incision was going to be on my lower right abdomen so based on that I brought a pair of comfy high waisted sweatpants to wear home (and by "home" I mean the ultra sterile Kidney Transplant House).
My only advice and I give it to everyone is whatever you (and your sister) do DON'T get constipated. Bowels have a tendency to sleep for several days after surgery so be very careful what you eat just before and after surgery. Your incisions will need time to mend without added strain. Walking a few times a day also helps.
My donors advice is that she wished she had gotten her incision side-to-side not up-and-down because it sometimes dents in slightly and she thinks if she had chosen sideways it wouldn't be visible at all.
Good luck. I'm so thrilled for you and your sister!