Instead of confiding in her about that, you could say something about what a busy day it has been, how fun it was getting ready in the dorm with the other girls, that you weren't sure what to expect, but it has been so much fun, etc.
I don't even really think you have to ask any questions unless you really need one answered. Don't be afraid to ask the sister, she wants to tell you about her sorority. Other than that just let the conversation flow!
Remember that just as important as the questions you ask is the way you answer questions! Don't mumble, give one-word answers or leave it up to the rusher to carry the conversation. I hate trying to rush a girl who just clams up. We need to talk about something for 10 minutes, and if you make it really difficult and unpleasant, you're not putting yourself in a good spot. Also, I won't know anything about you - not memorable. I love it when a PNM asks questions, because it means she's interested in the conversation (and it gives me a break)!