I think the other ladies have given you some great advice about what to talk about. I'm very proud of you for keeping an open mind about these two that asked you back. I know from personal experience at UNT that even if you don't get asked back, don't drop out! I have had more women become sisters out of those that didn't get asked back during my tenure than I can count on one hand. And those were some of the best women in the chapter!
Another thing you can ask, is what their first semester in the sorority was like? What was the pledge period like for them individually? What events do they enjoy the most in the sorority? What other community service does the chapter do besides their philanthropy. I know there are several on UNT that do more than just their national philanthropy.
You can also ask about sister sororities and campus involvement. Think more on a personal level with these girls than just chapter-wide. Each one of these girls joined their respective sororities for a reason and each one had things that touched them. Ask about these. Don't be too deep, you'll have time for that later, but show interest.
I remember the best advice i was given, If you can walk into a house and know beyond doubt that you can call it home, then you are in the right place.
Remember that these girls may become your sisters. Good luck! And, i'm rooting for cupcake.