Answer: I dont have a sister, but I do have a brother with the same illness, so hopefully this will help...you've got to have a real, honest, heart to heart conversation with her, and ask her why she acts this way. If she responds with one of her 6 year old responses, then you need to shake your head, then walk away slowly, (for dramatic affect). If she doesn't change within 24hrs then, there is one last thing to try. Secretly record her acting like a 6yr old and then show the recording to all of her six year old friends! The embarrasment will be so severe she will most likely stop.
Hope this helps!