I have to agree with most all these posts above. Hootie made excellent points. A legacy should never be a for sure thing. Once upon a time, I was part of a local sorority and the SAME exact thing happened. One of our most recent alumni had her sister rush the sorority and she bascially told all the other PNM's and sisters that she would get a bid. There were sisters that didn't think it was right, (including myself), but sadly when the time came around, she ended up getting a bid anyway. As a pledge (or new member as we call it now) she pretty much continued the same behavior and didn't carry her weight in the organization.. she still told everyone that she would be initiated because she was a legacy. In the end, she did get initated. but the semester after she did get iniated, she dropped from sight and we never saw her again.
Basically moral of the story - you need to do what is best for the entire organization. If she's not going to help with fundraisers, philantrophies and just sit around and look pretty in her "letters" I don't think she would be a great asset to your sorority's chapter. The local I used to belong to learned their lesson from that one case and has since started to view legacies just as the other PNM's and didn't treat them specially. ... I'm basically just rambling now, but you get my point