If your sister needs assistance in thinking up an idea, then I dont believe she chose the correct course. If shes not imaginative enough to comprise a simple story premise, then she will most probably be unable to extrapolate and sustain a storyline which has been GIVEN to her by one of the board members Besides, the extension two piece is supposed to revolve around and further explore a concept researched in either advanced English or extension 1. Without knowing the courses shes studied, our advice would be ultimately useless.
My advice is that she look over the course topics shes done in the past and choose her favourite topic. Then when shes chosen that topic, begin to consider what direction she could further explore this concept in, deciding whether to create a web-site, short story, or use any other medium in the wide range available to this course. When shes thought about which topic and which format would be most appropriate, then she can think about her idea for the actual work itself. I advise her to try and come up with her own concept though Otherwise shell never have the determination to complete something SHE has created.