I had no idea what age a "9th grader" translated to, so I googled and found "14 to 15". These are based on the assumption that is correct. And that your sister is like the majority of the kids around here and can type SMS messges with one hand, whilst conducting a conversation with another phone in the other hand :)

1. A "txt msg" to text translator.

   Take a text in the "Wat R U Doing L8R"; produce "What are you doing later."

2. A text to "txt msg" translator.

   Vice versa above.

3. An SMS message entry algorithm with predictive completion.

   Using the numeric keypad, the typical mapping and a dictionary.

One of these might just be familiar, topical and relevant enough to inspire her attention.