I like the kill her with kindness suggestions...
When she starts in, shower her with complements, tell her how proud you are of your baby sister. Ask her about her plan, let her tell you all about it. When she asks you how your diet is coming a along, just say real excitedly, "Oh really good". If she tries to pressure you into a number just say, "I really want it to be a big surprise for you when we see each other again, I think you'll be proud of me too"