I wouldn't worry about being a legacy, because the only mandate is that a legacy be invited back for Round 2. I hear about legacies getting cut all the time. From what I know, if a girl makes it to pref round she is guaranteed a bid on bid day. IN FACT, one of my friends only had Theta on her pref card so was guaranteed the bid lol.
This is true for some but not all sororities, FYI.