Hi Maria!  How sweet of you to want to do something special for your sister's graduation.  I assure you there are lots of little ways to show her you're proud of her without spending a lot of money.

First, when you arrive at your resort, make sure you ask for an "I'm Celebrating" button that she can wear the whole time your are there.  There is a space to write what she's celebrating, and often Cast Members will give a special greeting or congratulations to people wearing these pins.  If your resort does not have them, you can obtain one at Guest Services at any of the theme parks, free of charge.  Also, there are Mickey Ears Graduation hats that you could get her name embroidered on for a personalized touch.  If you would like to order a special dessert, at most table service restaurants you can order a small 6-inch celebration cake-no advance notice is required, and they are reasonably priced.  Finally, how about picking up some balloons, streamers and banners from your local dollar store before leaving home, and if she's out of the room, quickly decorate and surprise her when she returns.

Good luck with the planning, and I hope you have fun celebrating your sister's accomplishments!