After lots of talking, Matt and I finally came up with a plan that we thought would work out fine. We decided to blow up a blue balloon with helium and wrap it up as one of her presents. Then, at the end of the party (after she had already gotten all of the attention on her for most of the time!) we'd have her open her present. Since it was filled with helium, when she opened it, it would just float out and surprise everyone.
We decided to go ahead and tell my sister our plan because we knew that we would want someone to video tape the whole thing. So she totally knew everything (even about the upcoming ultrasound) weeks in advance. :)
It all turned out completely perfect! Here are some pics and the video from our announcement: