Write her a rec and keep in mind that the vocal minority last year are even more minor this year-- the majority of your sisters rushing on the floor will be from last year's pledge class.
It's up to your sister and the chapter from here. I understand your wanting what is best for her, but you can only do so much and she's going to have to go through some bumps in the road now that she is an adult. If your chapter is struggling numbers-wise, they will likely to do all they can to make those numbers. In this situation, pledging a legacy would probably be high on the list of priorities toward meeting this goal. In rhis case, there would be some pressure from your higher-ups to pledge interested legacies and pressure to not release them.
Take her to lunch or send her a special gift (maybe a bracelet) as a "Good luck" with recruitment send-off.

And best of luck to you both!