Rachel, remember that you're only going to be able to accept ONE bid, so don't stress out that you don't have the full compliment of invitations right now. You have 3 houses that you LOVE, and that's what's important.
What do you talk about? What does Rachel like? Just like everyone else has stressed, be yourself. You've shown yourself to be an outgoing, fun woman this far (and here on GC), so whatever you're doing, just keep it up.
I would suggest that, if you've talked to Sister A about something in particular, ask her about it to show that you remembered talking to her. Every example I can think of right now sounds contrived, but say that Sister A mentioned that she went to Florida for Spring Break last year. Ask her if she plans to go there again, where did she stay, would she recommend it - that sort of thing. Yeah, weird example, but I'm running on fumes...
Have fun, and be yourself!